Go to the bottom right side of the screen, look for a big box of colors. Select the part(s) you want to change, and then click the color you want them in. In case you don't know how to select multiple parts, click on one of them, then hold down command shift while selecting other parts. BTW you can also use this to move large sections of the build around your mom.

Oops. Sorry about that, meant your MOC, not mom. (autocorrect=my enemy)

This is a Lego modeling site, unless that's what you're here for, get out. BTW, I'm telling you this now before someone else does in a little more heated way.