"I will paint only what you tell me."
Translation:Je peindrai seulement ce que tu me diras.
You're correct that if the subordinate verb action is known to be taking place in the present, you use the present tense in French rather than the future. So if instead of "you tell me", it were "you are telling me (now)", then present tense works. But in this exercise, with "you tell me" and not "you are telling me", there's no indication that the action is taking place in the present, so the future tense should be used.
Yes, I fully agree that without additional context, it's ambiguous whether the telling action is taking place in the present or future. To clarify my original point: in such cases where you have 1) a main clause in the future tense and 2) a subordinate clause where it's ambiguous whether it takes place in the present or future, there's a grammatical difference between English and French in which verb tense is used for the subordinate clause. In English, the simple present tense is used by default with this ambiguity, whereas in French, the future tense is used by default with this ambiguity. In both languages, if additional context removes the ambiguity, then these default verb tenses can be overridden.