So if I make a multiplayer map, is it possible to host it online so people can play it?

I think you can. There's an option for it when making an online match, but I never got it to work. But that's most likely because I had problems with WWP online anyway.

So is it the same with custom maps?

So is it the same with custom maps?
You can even use color maps (bmp, jpg, tga) in your mission to play it online.
(Something you can't do in a normal online game.)

Multiplayer missions are 2-players only.
You also need the service pack 1 (patch) before you can use the mission editor.

Oh, great ^^
Also, can you have more than 64 colors in a map?
And how do you set photoshop to 64 colors only?