All the cows of the rainbow.

Blue cow?
Moo cow.

Red cow?
Dead cow.

Yellow cow?
Mellow cow.

Green cow?
Mean cow.

Indigo cow?
Where’d ‘e go, cow?

Violet cow?
File it, cow.

Orange cow?
Don’t be silly.
Everyone knows nothing rhymes with orange.

This took about five minutes - and it shows. I think it needs pictures. A rainbow for each couplet, with colours merging into the appropriate cow.

It was another flash - prompt 'blue cow'.