I don't know of a way to visually see all variables at once, but you can use the search tool to find them (On the Edit menu, click Find. In the Find drop-down box, click Any Variable, and then click Find).
Depending on how long you want the changes to appear, you may be able to use Find / Replace to apply a temporary character format that includes a color?
Use "As Is" for all options except the color that you want to apply to the variables. To return things to normal, use As Is for all options, then choose Black as the color...(?)
Not a built-in feature and so a bit clumsy, but it does achieve the result...(?)
Note, too, that you can Find variables by name if you wanted to apply unique colors to specific vars...
Cheers & hope this helps,
He may be too modest to plug it himself, so I will do it for him:
The $35.00 BookVars tool will help you keep you manage variables efficiently; especially if you work with multiple books.