On 2010-11-28 01:49, Jeff Kowalczyk wrote:

Color newbie: How do I keep a perceived color from changing when I add opacity? Is there a formula to use in RGBA space, or do one of the other color coordinate systems have this as one of their dimensions?

I have a specific logo color #00538FFF that I'm using in other graphics. For these other graphics, I have a front layer with a rectangle fill of this color, opacity set the 80%. The desired effect is that photos in lower layers become visible but clearly tinted in the main color.

The 80% opacity is clearly not a color match for the original RGBA #00538FFF, nor did I expect it would be.

My question is: how do I calculate the matching color at a certain opacity? ...

Does the color picker do what you want if you make sure to deselect the "Pick" toggle in the toolbar? (It says "Opacity:" and then has two toggle buttons, "Pick" and "Assign", make sure "Pick" is NOT selected/pressed down.) Or do you want to do the opposite, find a color that makes a transparent color visually match an opaque color? The latter is not possible in general, as the appearance of the transparent color depends on whatever is below it.

From what you're saying I think you may actually want to do a "Replace Hue" kind of operation. This can be done using filters, but is a bit more complicated. If you want to try, I have a filter available. It would also be possible to do something similar in an extension, but I don't think such an extension currently exists (it should work fine though, except that certain gradients may look a bit strange).