Adjustable color gradient with manual interpolation without using Processing's triangle strips.

GeneralRamp.pde

void setup() {
  size(800, 800, P2D);
}

void draw() {
  float[] positions = new float[]{0, 0.25, 0.5, 0.55, 1};
  int[] colors = new int[]{
    color(0), 
    color(4, 38, 102), 
    color(255, 255, 255), 
    color(68, 144, 201), 
    color(0)
  };
  ramp(positions, colors);
}

protected void ramp(float[] positions, int[] colors) {
  if (positions.length != colors.length) {
    throw new IllegalArgumentException();
  }
  for (int y = 0; y < height; y++) {
    int prevPosIndex = 0;
    int nextPosIndex = 1;
    float yNorm = norm(y, 0, height);
    for (int i = 1; i < positions.length; i++) {
      float pos = positions[i];
      if (yNorm >= pos) {
        prevPosIndex = i;
        nextPosIndex = i+1;
      }
    }
    float yNormalizedBetweenPrevAndNextPos = norm(yNorm, positions[prevPosIndex], positions[nextPosIndex]);
    int interpolatedColor = lerpColor(colors[prevPosIndex], colors[nextPosIndex], yNormalizedBetweenPrevAndNextPos);
    stroke(interpolatedColor);
    line(0, y, width, y);
  }
}