I agree with pretty much all that's been said, but here's my twopenny worth:

To attract attention you pretty much want the brightest colours (normally white / blue). These are also the best to read by, but they lose your night vision quickest. If night vision is important then in the UK I would definitely suggest a YELLOW light source. Whilst this is more invasive than a red or orange light, it has a big advantage over both when it comes to map reading. Both red and orange tend to lose the contours on Ordnance Survey maps (bad news); green will lose the forest and vegetation; blue the rivers, lakes and sea (more bad news). With yellow you only really lose the minor roads and they are still visible due to their black border.
Hope this helps.