Bases for sauces?

[eveglass]

I cooked a decent breakfast-for-dinner tonight, one of the few things I know how to cook consistently well. Simple but effective: scrambled eggs with onion and garlic, accompanied by baked beans on toast. Yum!

Doing all this cooking (ha.) made me think of a question: what sort of things are generally used as bases for sauces? So far I've come up with:
- milk and cream (white sauces)
- tomato (red sauces)
- gravy (brown sauces)

Is there anything else that can be used as the base for a sauce, or is that about it?

Thanks, cooking friends! I know you'll come through on this one!

Edited to add: I suppose something like a thick vegetable puree could be used as well, to make a... green sauce? Yes? No?