What about Čtyři barvy jsou určitě dost?
It is now accepted although "jsou dostatečné" is more clear. Most of the sentences with "... jsou dost." actually mean "... are/is a lot/plenty.".
Why can't you use jsou for are in this sentence?
Please always report the complete sentence. Note that the are is used in are enough, you can't translate the are on its own.
If you want something like Anders suggested, see the answer that will appear under his question.