Your LED sign is made up of several little lights. The LED lights are separated into three colors. Red, Blue, and Green. By changing the brightness on these lights your sign can go from black to white. Changing the brightness on just one color or only two your sign can become any color in the rainbow. Each set of three LED lights is called a Pixel. This is why you build messages at your signs pixel width and height so that the pixels line up.
When using color on your LED sign it is important, maybe even more so than in print, to use contrasting colors. because LED signs are dealing with brightness a yellow background with white text would be very hard to read, if at all. So some tips on keeping your sign noticed and readable are:
1) Use contrasting colors
2) Have a contrasting background with text (ex: blue background with yellow text.)
3) Stay away from monochromatic color schemes (ex: yellow, orange, and red)
4) When you need to use the same colors, try putting a border, or shadow around text.
5) When putting text on an image with multiple colors, try putting a solid color box in between the image and your text.