[lcrs]

These actually increase the apparent contrast you see on the screen. They're especially helpful for LCD panels which have washed-out and uneven black which is very noticeable in a dark room. Having the colours match the picture seems largely a gimmick to me, but having a light surround makes the screen appear "darker". The exact effect it has is hard to measure, and attempts have been ongoing since the 50s. Here's a recent paper: http://www.scopecalc.com/image-contrast-and-surround-illumin...

There are SMPTE specs for exactly how much of what colour light should be used, and we follow them at work, but at home a string of LED fairy lights behind the screen helps a lot ;)