Once you have a part, all you have to do to use it is go to the Part menu and select "Add from File" and pick the .SUF file you want...
DoGA can write BMP files if you tell it to render to file, I think; I don't remember how to do that right now. To change the color of a part, select it and double click on the colored rectangle near the lower left corner of the screen with some sort of generic description (I think it's "material1") and you get a wealth of color and texture objects - you can even make translucent and anime shaded objects!
If you want multiple colors on different parts, you'll need to add a new material; I think there's a button near the lower right that lets you do that. (Sorry, I don't have DoGA with me at the moment so I can't look more closely at it.)