COLOR <background code><foreground code>
0 = Black 8 = Gray 1 = Blue 9 = Light Blue 2 = Green A = Light Green 3 = Aqua B = Light Aqua 4 = Red C = Light Red 5 = Purple D = Light Purple 6 = Yellow E = Light Yellow 7 = White F = Bright White
For example, to get a white on blue screen type:
COLOR 17
Of course, if you rather, you could press ALT+SPACE and select "Properties | Colors" to change colors as well.