Posted my problem a few days ago (purple and green alternately taking over the colour spectrum when avi movies were converted with TMPGEnc) and received several very helpful replies.

I did two of the things suggested so don't know which solved the problem.
One member told me to uninstall Divx 4.12, then install Divx 3.11 and after that reinstall Divx 4.12 but not to allow it to be the code for ALL Divx files when asked at the prompt.

Another member told me to go in to the Options/Environmental settings/Vfapi in TMPGEnc and change the priority of the Direct show multimedia file reader to 2 instead of the default.

I did this and now get ordinary colours again.

Thanks to all who replied!