State government agencies can be as brand/model-specific as they want when they are specifying and opening bids for the vehicles they purchase and own. However, state governments are not legally able to approve or disapprove, require or prohibit equipment or design features on vehicles they don't own, if that equipment or design feature is subject to a Federal standard. Any such approval/disapproval/requirement/prohibition is legally null and void because of the Federal preemption.
State governments can regulate usage of equipment, however. They have the authority to say when high beam headlamps may be used, when low beams must be used, etc. And state governments can regulate equipment that isn't subject to Federal standards -- for example, a state could decide fog lamps may not be used on public roads, period.
But any state list of "approved" turn signals, headlamps, brake lights, or whatever isn't worth the paper it's printed on. It is of zero legal weight.