Choosing colours to be "reasonably" distinguishable can be done by representing the colours on a colour wheel and using the angle between the colours as a measure of distance. Google gives me this page which might be a start, and there is also Graphics::ColorObject, which has the HSV (Hue, Saturation, Value), which should have the hue as the angle.

The n high-contrast values for the colour c are then the colours located at the angles c + k * 360/n for k between 0 and n - 1. This will obviously only work for a small number of "contrasting colours", as the segments become too small soon to be still regarded as contrasting if you allow arbitrary ordering of the colours.

If you only want local contrast as opposed to global contrast, then things become harder, as you now have to offer colours that have high contrast to all touching colours, and you'll have to find out which flag regions are touching the current flag region to restrict your colour selection offers.

Update: chromatic points out that the V in HSV stands for "Value".

In reply to Re: Choosing Colors for Custom Flags by Corion
in thread Choosing Colors for Custom Flags by dimar

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