My Goodmans TV turns black and white after a while?

My TV will turn on in colour. As soon as the channel is changed the screen displays black and white. However the on screen display (e.g. channel numbers) are still green. To get the tv back in colour at the moment I need to switch to the desired channel, turn the TV off for about 5 minutes and then turn it back on.

I am fairly competent at soldering and have no problems with opening up the tv, but I wouldn't mind a little guidance as I have never fixed a tv before.

Thank you

Sach