MonoGame.Samples/Shaders2D/Core/Content/effects/RainbowH.fx

sampler s0;

float4 red    = float4(1,0,0,1);
float4 orange = float4(1,.5,0,1);
float4 yellow = float4(1,1,0,1);
float4 green  = float4(0,1,0,1);
float4 blue   = float4(0,0,1,1);
float4 indigo = float4(.3,0,.8,1);
float4 violet = float4(1,.8,1,1);

float step = 1.0/7;

float4 PixelShaderFunction(float2 coords: TEXCOORD0) : COLOR0
{
    float4 color = tex2D(s0, coords);

	if (!any(color)) return color;

	if      (coords.x < (step * 1)) color = red;
	else if (coords.x < (step * 2)) color = orange;
	else if (coords.x < (step * 3)) color = yellow;
	else if (coords.x < (step * 4)) color = green;
	else if (coords.x < (step * 5)) color = blue;
	else if (coords.x < (step * 6)) color = indigo;
	else                            color = violet;

    return color;
}

technique Technique1
{
    pass Pass1
    {
        PixelShader = compile ps_3_0 PixelShaderFunction();
    }
}