Here's the problem. I prefered to report it here so I could attach the pictures in order.
When I first ran FS2_open with the gamma code everything was too dark. I used the brightness option to make everything look good again.
When I quit out of FS2 everything has gone dark again. I suppose it's worth mentioning that Windows is used to be very dark for me anyway. I adjusted the settings using the control panel that came with the catalyst drivers until windows was acceptably bright. Here's a pic with my settings the way I like them.
The windows defaults were Gamma : 1 and Brightness : 0 and were far too dark.
When I quit out of FS2 my desktop is at the unadjusted settings. If I click on display settings-->advanced--->colour to bring up the panel above my settings instantly change to the ones I like (but they only remain if I click okay rather than cancel).
If after doing this I then play a different game I find that it is too dark too. If I ALT-TAB out of this game call up the colour panel and click okay the game looks fine. My 3D colour panel has also been adjusted because games were also too dark until I adjusted the colour. My 3D panel looks like this

If I play FS2 again after quiting everything looks fine in-game but I get the same problem on quitting again.
My stats are as follows.
AMD XP 2100+
A7N8X Deluxe
Radeon 9800
1Gig DDR