Retinal Fields for Colours?

When I use a centre-surround field to detect blobs, it does not see apples in a tree
I think this is because I've transformed the input to grey-scale
Because of this red and green are pretty much the same intensity and therefore a red apple with a green background exhibits no contrast
But what if fields compared different colours?
Could contrast the primaries:
  Red-green - this would see apple in tree
  Red-Blue - this would see autumn trees against sky
      Red is dirt, fur, fire, flowers
  Green-blue - this would see trees against sky
Blue is good for shadows, water, sky
Also white and black against primaries:
   Black is night and shadow, also fur, rocks
  Grey is rocks, clouds, shadows, tree trunks, elephants
  White is snow, clouds, reflectance, flowers
This way I'd get more blobs, with specialization, to provide expectations. I'll try it.