I only recall the BtS HUD, which is blue. DoC actually doesn't have custom interface colours unless you install the Carbon HUD module, so it is identical to BtS. Was vanilla different? Or are you thinking of something else maybe?
Edit: the vanilla interface config file is not compatible with DoC so I couldn't try, but I did look up the color defined there, which is
RGB 69 93 156, the same blue.
In any case, if you want to change the interface colours or use those of another mod, that should be pretty easy too. The best approach is to start by installing the Carbon HUD module. Then:
1) changing the main background colour only:
Open Resource/Themes/Civ4/Civ4Theme_Common.thm, search for ".BG" (you should find a "GColor .BG" entry) and enter the colour in the GColor entry next to it. The four values are RGBA (red, green, blue, alpha). Alpha is transparency which you can ignore (unless you want a transparent HUD for some reason?), RGB values are 0-255 decimal numbers. The Google colour picker I linked to above is a good way to find the right values for your desired colour.
2) steal another mods colours:
This is a slightly better approach because a theme consistens of more than just the main panel background colour, e.g. buttons and texts can have their colours changed too in appropriate ways. To do so, simply delete the entire Resource folder and replace it with the Resource folder from the mod whose interface you want to use.