If you want to maximize contrast, pick a background which is opposite on the colour wheel to your indicator. So white would be black, orange would be blue. Dark grey and black works pretty well too, though it breaks this rule. Enter a colour and choose 'complimentary', or enter a specific RGB. It might sound like a small thing, but having your charts perfect for viewing is pretty essential. You've got to be able to grasp what's going on with one glance.
