Gamemaker shader confusion

flash_red.gml

if red > 0 
{	
	shader_set(sh_red);
	
	shade_alpha = shader_get_uniform(sh_red, "alpha");
	shader_set_uniform_f(shade_alpha, 0);

	draw_self();
	shader_reset();
}

player_draw.gml

draw_self();

flash(0.05); // This is unrelated.
flash_red();

sh_red.fsh

//
// Simple passthrough fragment shader
//
varying vec2 v_vTexcoord;
varying vec4 v_vColour;

uniform float alpha;

void main() 
{
	vec4 color = texture2D(gm_BaseTexture, v_vTexcoord);
	
	if (color.a != 0.0) 
	{
		gl_FragColor = vec4(1, 0, 0, alpha); // This tutorial that I watched showed that setting the first number to 1 made the object completely red, though for whatever reason it has not affected the look of the player at all.
	} 
}