"Her lipstick is blood red."
Translation:Son rouge à lèvres est rouge sang.
Both words are adjectives, neither of which would precede a noun however, in this sentence, one is being used to describe the other.
What colour is the lipstick? It is red. What shade of red? Blood red.
As "rouge" is the colour being described it precedes "sang".
The other rouge is part of the masculine noun "rouge à levres" meaning lipstick.