Dominant means the homozygous mutant and the heterozygous mutant are identical (e.g. spiders). However, it's difficult to prove a gene to be for certain dominant, since you might just be getting bad odds. You have to breed a homoygous to several normals and get 100% morph offspring to be certain. To date actually, nobody has proven a homozygous spider.

A co-dominant gene on the other hand is where the heterozygous form and the homozygous form are completely different (like mojave and leucistic).

The pastel gene on the other hand is actually incomplete dominant rather than codominant... which is when the heterozygous form is an intermediate between the normal and the homozygous morph i.e. a pastel is basically just part way between a normal and a super pastel. Although for the purposes of the herp industry, the subtle difference between incomplete dominant and codominant isn't really important and it's easier to just call them all co-dominant since it's shorter.