I think if you are wanting to do this then you shouldn't go and change the actual variables themselves it would be best to do it dynamically. i.e. get each script to look at the time and then put the appropriate colour variables into it.

It is extremely complicated though unless you know every visitor to your site is in the same part of the world, to work properly you would have to get them to input their latitude and longditude and correct current timezone and have access to forumulas which will calculate sunrise and sunset times for everywhere around the world - not an easy task.