Curious. Probably there's a bug on the form where the field it's complaining about isn't currently visible. You could try popping open the various form sections to see if there are any that are showing an error.
Does this happen always when you edit the theme settings, or just when you make particular changes?
Joe
Yes, you were right - i found a no deleted image in the header section even though i had selected primary colour option.
clearing this section has appeared to resolved the issue.
thanks :)