I agree that something like this is probably much better than what I'm doing in LDView. Having said that, I don't think adding (or subtracting) a constant value to the RGB components of the color and then clamping is the best way to do it. It seems that would change the hue noticeably.

For example, suppose that the input color was #FF8040 (kind of tangerine orange). Your algorithm would produce #991A00, which is more of a dark red. Multiplying each component by 0.6 would produce #994D26, which is brown. Generally, "dark orange" means "brown", so I think this is better.

Going the other way (creating a light compliment for a dark color) is a little more work, since you have to pull each component towards 1.0 by 0.6, but it's still fairly simple. In my opinion, the result is a better match of the original color.