I may have misunderstood your last post but the defaults for sprites is to have anything black not drawn to the screen. So if you were to have an image that contained just one letter and a black background, when you loaded the image in a sprite it would just draw the letter.
To get the effect i think you want, first use a sprite to draw/load the rainbow, then use sprites to draw/load the letters, you should just see your letters with a rainbow background.
If you want black letters, use MS Paint or an editor of your choice and still have a black background - RGB(0,0,0) but colour the letter RGB(0,0,1). You wont see the letter in the editor but in DB you will.
The harder bit is to position the sprites correctly on the screen and scale the rainbow to cover just the letters, no more no less.
If the above is what you need, and require any more help on any of the above, i should be able to help you all the way
James Morgan