(Note: This is alot simpler if you have only a few different colours!)
A nice border effect is one where the second innermost pixel border is highlighted, like:
To get this effect, it is necessary to use the HUE, SAT, LUM settings in Colours > Edit Colours.,These are just an alternative method from RGB of determining a colour (Note: HSL goes from 0 to 240 rather than 0 to 255, thus less colours are possible, but 240 is chosen because it has many convenient factors, e.g. 2, 3, 4, 5, 6, 8, 10, etc). We need to increase the brightness of the colour, so this is LUMINOSITY. We want to make the colours THE SAME AMOUNT CLOSER TO FULL, so to do this, some fairly simple Maths is required:
Taking the value of the luminosity of the original colour, subtract this value from 240 (e.g. if original was 168, value becomes 72). Now, multiply this value by a fraction either less than 1 for increasing brightness, or more than 1 for decreasing (e.g. in a light image). If the fraction was ½, then this result would give 36. Take this away from 240 to give the new luminosity (204).
Do this on every pixel, and you can get an effect just like the one above!