Background Design (Gimp Users)

I want to make good backgrounds for my game that includes shades, blends and a lot of color; however I want to give the game a selected limited color palette (260 Color Max). This is the problem, all of Windows 7's Microsoft Paint's and Gimp's tools I've seen so far besides pencil and paint-fill disobeys that rule by having auto shading and blending. I want to know for those who use paint programs like Gimp to make background design, if there is an option to use a limited color palette (with the colors of your choice) and keep it at its limit (That means no artificial blending).

Examples: Say I want to use two colors from my custom palette that are right next to each other (white and a very bright pink). Since the colors are right next to each other in the palette, they shouldn't blend in no matter what.

Say I want to use two colors from my custom palette that have many other colors between them (Red and Blue). I do want them to blend, but I want the program to only borrow the colors from that custom palette for blending and nothing else (All the purples I see in the blend, I can see in my custom palette).

Sorry I could not explain this any more than I should.
P.S- Multimedia Fusion’s 256 Color limit is out of the question, because its palette cannot be edited.

You can have an active object with your 256 color in the mfa and edit it when you need any of the colors.

Marv