When you make a colour partly transparent, it lightens (assuming it's on a white background), naturally. I would like to counter this behaviour by "darkening" a colour, so that when I make it transparent, when on a white background, the end result is the same as the original colour.
Basically, I would like to compensate the effect of transparency. And I have no idea of how I would do that. I've tried playing with some numbers in the colour's parameters, but I haven't been able to find what I should do.
This might be hard to understand in words alone, so I've made a visual explanation of what I would like to achieve.
What I want to achieve is the highest square.
Oh, before a clever one tells me to simply do what I did on that image, of course the example was faked by cutting a piece of transparent orange to put over the area where the "stick" overlaps with a solid-coloured square. Of course, that solution can't be applied to more irregular or changing shapes, and won't handle the white areas of a picture.
In case it's important, what I'm trying to make with this is a web design in Adobe Illustrator, which would of course eventually end up being made out of HTML and CSS.