My first attempt at "seriously" using GX2.

CMakeLists.txt

cmake_minimum_required(VERSION 3.2)
project(glitchy-rectangle C)
include("${WUT_ROOT}/share/wut.cmake" REQUIRED)

add_executable(glitchy-rectangle
   glitchy-rectangle.c)

target_link_libraries(glitchy-rectangle
   whb
   gfd
   gx2
   proc_ui
   nsysnet
   sysapp)

wut_create_rpx(glitchy-rectangle.rpx glitchy-rectangle)

glitchy-rectangle.c

#include <gfd.h>
#include <gx2/draw.h>
#include <gx2/shaders.h>
#include <gx2/mem.h>
#include <gx2/registers.h>
#include <gx2r/draw.h>
#include <gx2r/buffer.h>
#include <string.h>
#include <stdio.h>
#include <whb/file.h>
#include <whb/proc.h>
#include <whb/sdcard.h>
#include <whb/gfx.h>
#include <whb/log.h>
#include <whb/log_udp.h>

#include <coreinit/systeminfo.h>
#include <coreinit/thread.h>
#include <coreinit/time.h>

#include <stdlib.h>

int main(int argc, char **argv)
{
   GX2RBuffer positionBuffer1 = { 0 };
   GX2RBuffer positionBuffer2 = { 0 };
   GX2RBuffer colourBuffer1 = { 0 };
   GX2RBuffer colourBuffer2 = { 0 };
   WHBGfxShaderGroup group = { 0 };
   void *buffer = NULL;
   char *gshFileData = NULL;
   char *sdRootPath = NULL;
   char path[256];
   int result = 0;

   srand(OSGetTick());

   WHBLogUdpInit();
   WHBProcInit();
   WHBGfxInit();

   if (!WHBMountSdCard()) {
      result = -1;
      goto exit;
   }

   sdRootPath = WHBGetSdCardMountPath();
   sprintf(path, "%s/wut/content/pos_col_shader.gsh", sdRootPath);

   gshFileData = WHBReadWholeFile(path, NULL);
   if (!gshFileData) {
      result = -1;
      WHBLogPrintf("WHBReadWholeFile(%s) returned NULL", path);
      goto exit;
   }

   if (!WHBGfxLoadGFDShaderGroup(&group, 0, gshFileData)) {
      result = -1;
      WHBLogPrintf("WHBGfxLoadGFDShaderGroup returned FALSE");
      goto exit;
   }

   WHBGfxInitShaderAttribute(&group, "aPosition", 0, 0, GX2_ATTRIB_FORMAT_FLOAT_32_32_32_32);
   WHBGfxInitShaderAttribute(&group, "aColour", 1, 0, GX2_ATTRIB_FORMAT_FLOAT_32_32_32_32);
   WHBGfxInitFetchShader(&group);

   WHBFreeWholeFile(gshFileData);
   gshFileData = NULL;

   // Set vertex position
   positionBuffer1.flags = GX2R_RESOURCE_BIND_VERTEX_BUFFER |
                          GX2R_RESOURCE_USAGE_CPU_READ |
                          GX2R_RESOURCE_USAGE_CPU_WRITE |
                          GX2R_RESOURCE_USAGE_GPU_READ;
   positionBuffer1.elemSize = 4 * 4;
   positionBuffer1.elemCount = 3;
   GX2RCreateBuffer(&positionBuffer1);

   positionBuffer2.flags = GX2R_RESOURCE_BIND_VERTEX_BUFFER |
                          GX2R_RESOURCE_USAGE_CPU_READ |
                          GX2R_RESOURCE_USAGE_CPU_WRITE |
                          GX2R_RESOURCE_USAGE_GPU_READ;
   positionBuffer2.elemSize = 4 * 4;
   positionBuffer2.elemCount = 3;
   GX2RCreateBuffer(&positionBuffer2);

   // Set vertex colour
   colourBuffer1.flags = GX2R_RESOURCE_BIND_VERTEX_BUFFER |
                        GX2R_RESOURCE_USAGE_CPU_READ |
                        GX2R_RESOURCE_USAGE_CPU_WRITE |
                        GX2R_RESOURCE_USAGE_GPU_READ;
   colourBuffer1.elemSize = 4 * 4;
   colourBuffer1.elemCount = 3;
   GX2RCreateBuffer(&colourBuffer1);

   colourBuffer2.flags = GX2R_RESOURCE_BIND_VERTEX_BUFFER |
                        GX2R_RESOURCE_USAGE_CPU_READ |
                        GX2R_RESOURCE_USAGE_CPU_WRITE |
                        GX2R_RESOURCE_USAGE_GPU_READ;
   colourBuffer2.elemSize = 4 * 4;
   colourBuffer2.elemCount = 3;
   GX2RCreateBuffer(&colourBuffer2);

   WHBLogPrintf("Begin rendering...");
   while (WHBProcIsRunning()) {

      float topLeftCornerX = -0.4f - ((rand() % 900) / 10000.0f);
      float topLeftCornerY = 0.4f + ((rand() % 900) / 10000.0f);

      float topRightCornerX = 0.4f + ((rand() % 900) / 10000.0f);
      float topRightCornerY = 0.4f + ((rand() % 900) / 10000.0f);

      float bottomRightCornerX = 0.4f + ((rand() % 900) / 10000.0f);
      float bottomRightCornerY = -0.4f - ((rand() % 900) / 10000.0f);

      float bottomLeftCornerX = -0.4f - ((rand() % 900) / 10000.0f);
      float bottomLeftCornerY = -0.4f - ((rand() % 900) / 10000.0f);

      float topLeftCornerR = 1.0f - ((rand() % 30) / 100.0f);
      float topLeftCornerG = 0.0f + ((rand() % 30) / 100.0f);
      float topLeftCornerB = 0.0f + ((rand() % 30) / 100.0f);

      float topRightCornerR = 0.0f + ((rand() % 30) / 100.0f);
      float topRightCornerG = 1.0f - ((rand() % 30) / 100.0f);
      float topRightCornerB = 0.0f + ((rand() % 30) / 100.0f);

      float bottomRightCornerR = 0.0f + ((rand() % 30) / 100.0f);
      float bottomRightCornerG = 0.0f + ((rand() % 30) / 100.0f);
      float bottomRightCornerB = 1.0f - ((rand() % 30) / 100.0f);

      float bottomLeftCornerR = 0.0f + ((rand() % 30) / 100.0f);
      float bottomLeftCornerG = 1.0f - ((rand() % 30) / 100.0f);
      float bottomLeftCornerB = 0.0f + ((rand() % 30) / 100.0f);

      float * pos1 = (float *)GX2RLockBufferEx(&positionBuffer1, 0);
      pos1[0]  = topLeftCornerX;
      pos1[1]  = topLeftCornerY;
      pos1[2]  = 0.0f;
      pos1[3]  = 1.0f;
      pos1[4]  = topRightCornerX;
      pos1[5]  = topRightCornerY;
      pos1[6]  = 0.0f;
      pos1[7]  = 1.0f;
      pos1[8]  = bottomRightCornerX;
      pos1[9]  = bottomRightCornerY;
      pos1[10] = 0.0f;
      pos1[11] = 1.0f;
      GX2RUnlockBufferEx(&positionBuffer1, 0);

      float * pos2 = (float *)GX2RLockBufferEx(&positionBuffer2, 0);
      pos2[0]  = bottomRightCornerX;
      pos2[1]  = bottomRightCornerY;
      pos2[2]  = 0.0f;
      pos2[3]  = 1.0f;
      pos2[4]  = bottomLeftCornerX;
      pos2[5]  = bottomLeftCornerY;
      pos2[6]  = 0.0f;
      pos2[7]  = 1.0f;
      pos2[8]  = topLeftCornerX;
      pos2[9]  = topLeftCornerY;
      pos2[10] = 0.0f;
      pos2[11] = 1.0f;
      GX2RUnlockBufferEx(&positionBuffer2, 0);

      float * col1 = (float *)GX2RLockBufferEx(&colourBuffer1, 0);
      col1[0]  = topLeftCornerR;
      col1[1]  = topLeftCornerG;
      col1[2]  = topLeftCornerB;
      col1[3]  = 1.0f;
      col1[4]  = topRightCornerR;
      col1[5]  = topRightCornerG;
      col1[6]  = topRightCornerB;
      col1[7]  = 1.0f;
      col1[8]  = bottomRightCornerR;
      col1[9]  = bottomRightCornerG;
      col1[10] = bottomRightCornerB;
      col1[11] = 1.0f;
      GX2RUnlockBufferEx(&colourBuffer1, 0);

      float * col2 = (float *)GX2RLockBufferEx(&colourBuffer2, 0);
      col2[0]  = bottomRightCornerR;
      col2[1]  = bottomRightCornerG;
      col2[2]  = bottomRightCornerB;
      col2[3]  = 1.0f;
      col2[4]  = bottomLeftCornerR;
      col2[5]  = bottomLeftCornerG;
      col2[6]  = bottomLeftCornerB;
      col2[7]  = 1.0f;
      col2[8]  = topLeftCornerR;
      col2[9]  = topLeftCornerG;
      col2[10] = topLeftCornerB;
      col2[11] = 1.0f;
      GX2RUnlockBufferEx(&colourBuffer2, 0);

      // Render!
      WHBGfxBeginRender();

      WHBGfxBeginRenderTV();
      WHBGfxClearColor(1.0f, 1.0f, 1.0f, 1.0f);
      GX2SetFetchShader(&group.fetchShader);
      GX2SetVertexShader(group.vertexShader);
      GX2SetPixelShader(group.pixelShader);

      GX2RSetAttributeBuffer(&positionBuffer1, 0, positionBuffer1.elemSize, 0);
      GX2RSetAttributeBuffer(&colourBuffer1, 1, colourBuffer1.elemSize, 0);
      GX2DrawEx(GX2_PRIMITIVE_MODE_TRIANGLES, 3, 0, 1);

      GX2RSetAttributeBuffer(&positionBuffer2, 0, positionBuffer2.elemSize, 0);
      GX2RSetAttributeBuffer(&colourBuffer2, 1, colourBuffer2.elemSize, 0);
      GX2DrawEx(GX2_PRIMITIVE_MODE_TRIANGLES, 3, 0, 1);

      WHBGfxFinishRenderTV();

      WHBGfxBeginRenderDRC();
      WHBGfxClearColor(1.0f, 1.0f, 1.0f, 1.0f);
      GX2SetFetchShader(&group.fetchShader);
      GX2SetVertexShader(group.vertexShader);
      GX2SetPixelShader(group.pixelShader);

      GX2RSetAttributeBuffer(&positionBuffer1, 0, positionBuffer1.elemSize, 0);
      GX2RSetAttributeBuffer(&colourBuffer1, 1, colourBuffer1.elemSize, 0);
      GX2DrawEx(GX2_PRIMITIVE_MODE_TRIANGLES, 3, 0, 1);

      GX2RSetAttributeBuffer(&positionBuffer2, 0, positionBuffer2.elemSize, 0);
      GX2RSetAttributeBuffer(&colourBuffer2, 1, colourBuffer2.elemSize, 0);
      GX2DrawEx(GX2_PRIMITIVE_MODE_TRIANGLES, 3, 0, 1);

      WHBGfxFinishRenderDRC();
      WHBGfxFinishRender();
   }

exit:
   WHBLogPrintf("Exiting...");
   GX2RDestroyBufferEx(&positionBuffer1, 0);
   GX2RDestroyBufferEx(&positionBuffer2, 0);
   GX2RDestroyBufferEx(&colourBuffer1, 0);
   GX2RDestroyBufferEx(&colourBuffer2, 0);
   WHBUnmountSdCard();
   WHBGfxShutdown();
   WHBProcShutdown();
   WHBLogUdpDeinit();
   return result;
}