I see it as you describe, denzilquixode. I tried it with and without the XP Theme stuff (which I'm not familiar with), using a manifest made by Faena AddXPStyle like I normally use and then Khomy Prime's manifest, and in all cases only the tab portion itself shows as white, not the tab's main body:

I tried not setting the WINDOW_HIDDEN flag in case that might be throwing the color off, but that made no difference. I also tried parenting it to a panel in case it might inherit the panel's color (since a panel is one of the few gadgets that SetGadgetColor actually works with), but no luck.
XP Pro SP2, Radeon 9600 Pro in case it matters.

I tried not setting the WINDOW_HIDDEN flag in case that might be throwing the color off, but that made no difference. I also tried parenting it to a panel in case it might inherit the panel's color (since a panel is one of the few gadgets that SetGadgetColor actually works with), but no luck.
XP Pro SP2, Radeon 9600 Pro in case it matters.