uniform half4 colorGreen, colorRed;

half4 main(float2 coords) {
    float4 value = colorGreen.gggg * 2.5;
    float4 whole, fraction;
    bool4  ok;

    // 2.5 equals 2 + 0.5.
    fraction.x    = modf(value.x,    whole.x);
    ok.x          = whole.x == 2 && fraction.x == 0.5;
    fraction.xy   = modf(value.xy,   whole.xy);
    ok.y          = whole.y == 2 && fraction.y == 0.5;
    fraction.xyz  = modf(value.xyz,  whole.xyz);
    ok.z          = whole.z == 2 && fraction.z == 0.5;
    fraction.xyzw = modf(value.xyzw, whole.xyzw);
    ok.w          = whole.w == 2 && fraction.w == 0.5;

    return all(ok) ? colorGreen : colorRed;
}