% Use a uniform to prevent the compiler from optimizing.

[pixel shader]
uniform float i;
float4 main() : SV_TARGET
{
    float4 color = float4(0.5, 0.4, 0.3, 0.2);
    color.g = color[i];
    color.b = 0.8;
    return color;
}

[test]
uniform 0 float 2
draw quad
probe (0, 0) rgba (0.5, 0.3, 0.8, 0.2)