The "colourify" function in The GIMP simply does a desaturate (convert to grayscale), followed by a colour multiplication. Very simple. This should be the equivalent HLSL (untested):
float4 original = tex2d(...);
float q = (original.r + original.g + original.b) / 3;
q /= original.a; // optional - undo alpha premultiplication
return float4(tint.rgb * q, tint.a * original.a);
But you could achieve the same effect by simply storing all your textures desaturated to begin with and using the default SpriteBatch
shader for its multiplication. If you don't want to modify your source art, you could do it in a custom content processor.
Although, if you want to use a custom shader, you can do something more sophisticated. You could implement a full hue-rotation (kinda complex). But perhaps you could consider something like the "Overlay" blend mode (very simple) - which lets you colourize the grays, while preserving both the highlights and lowlights (instead of multiply, which also colourizes the highlights).