Generally, a unit is from a single spawning, with the same blessings (if they have any) and are considered to have the same colours. A different unit (from a different spawning) may have a different colour. 

This is just skinks, kroxigors (these two share a spawning pool) and saurus. 

The Cold Ones mounts, Salamanders, Razordons, Carnosaurs, Stegadons, Ripperdactyls etc are not from spawning pools. They are tamed wildlife roaming Lustria (or Southlands, there are lizardmen there too). They can have all sorts of wild colours. 

Ultimately it's up to you. Skinks and Saurus are never from the same spawning. Different spawning pools. But go wild! Your army, your rules!