It'd be cool if we could click a color in the palette, change it, and if that old color existed in the current icon, then have it change every color to the new color in the palette like we used to be able to do.

For example, an icon uses the color #00FF00. We doubleclick that color in the palette, change it to #059600 in the prompt following the doubleclick, and for every color in the icon that equals #00FF00, change it to #059600.

It was so incredibly useful, especially when you're testing the color with a TON of icon states.
I agree. I was sort of disappointed when I saw that you couldn't do this anymore in 4.0.
In response to QMarkReal
I super agree. Bring it back, yo~

Quote from Tom: "For now, you should use "swap". In the future, we can provide either 1) a toggle to enter into palette mode, or 2) more obvious swapping routines (including a global swap across all icons, which is particularly useful)."