Is HSV Color Space Sufficient for Rudimentary Color ID and Edge Detection
The approach seems reasonable.
Indeed doing edge detection in weighted RGB channel is the classic approach (Though you could also employ more advance methods, See Edge Detection on a Color Image).

I think you could achieve great results if you also look specifically for oval shapes then you reduce the chances for false positives. 

Color identification in the H and S channels will give most information (Well, up to differences in luminosity of the color).