The sentence "Which color do you feel suits you best?" is grammatically correct.

The word "do" in this sentence is an auxiliary verb used to form a question in English. In this case, it is part of the construction used to make a question when the main verb is in the simple present tense. The structure is as follows:

Subject + Do/Does + Main Verb (base form) + Object?

In this sentence:

"Which color" is the subject

"do" is the auxiliary verb

"you" is the subject of the verb "feel"

"feel" is the main verb (in its base form)

"suits" is the main verb (in its base form) of the second verb phrase

"you" is the indirect object

"best" is an adverb modifying the verb "suits"

So, the sentence is correctly formed as a question in the simple present tense, with "do" helping to form the question and "feel" and "suits" being the main verbs expressing the action.