Grep and --color

[Jori Mantysalo]

If I say
 echo "aaaab" | grep -E --color=auto "((aaa)|(aa))*"
I get four red-colored a-letter, but with command
 echo "baaaab" | grep -E --color=auto "((aaa)|(aa))*"
I just get gray text. So, grep finds match, but doesn't always color it.

What are the rules when matching with --only-match ? When I said
  echo "aaaa" | grep -E --color=auto "((aaa)|(aa))"
I got four red-colored letter, but then when I said
 echo "aaaa" | grep -E --color=auto -o "((aaa)|(aa))"
I got three red-colored letter and not two lines with two a:s in both.
So, is grep greedy or not when matching?

-- 
Jori Mäntysalo