[Asooka]

Also, you must work in linear colour space. In OP's case you would have to apply the inverse sRGB transform, do the calculation, then apply the forward sRGB transform. Best if you just work with linear colour end to end, if you have the memory to spare.

[astrange]

Well, it's certainly traditional to get that wrong. Technically nothing's wrong in graphics as long as you get some kind of image out of it, it's just different :)