Loss of colours

My next Sam physical hardware question — is there any reason why my  
Sam might find itself merging similar colours? Check out:

http://members.allegro.cc/ThomasHarte/temp/Image(072).jpg

That's how the first screen of Prince of Persia looks from both the  
SamCo Birthday disk and Blitz 8 (thought I'd check, since the birthday  
disk one is clearly an earlier version of the demo and, for all I  
knew, predates the game being completed). It's not a great photograph,  
but it does demonstrate the point. The walls on the TV are as they  
appear in that photo, a single solid blue. The effect is the same  
through both the aerial socket and RGB SCART, with or without the  
Trinity plugged in. I have no other hardware attached.