Yes, you can absolutely get a street bench with different color slats than the frame. In fact, this is one of the easiest ways to customize a bench without going fully custom. It is often called a two-tone bench finish, and it can give a standard streetscape, park, campus, or storefront a much more designed look.
Most commercial bench manufacturers treat the frame and slats as separate finish decisions. The frame is usually cast iron, steel, or aluminum with a powder-coated finish, while the slats can be metal, wood, recycled plastic, or composite. Because those parts are finished separately, mixing colors is very common. A black frame with natural wood slats is a classic combination, but you are not limited to that. You can choose gray slats on a green frame, blue slats on a black frame, or even brighter combinations like red or yellow slats on a neutral frame.
If you are ordering a standard catalog bench, check the product page for frame color and slat color dropdowns. Many brands list them separately. If the combination you want is not shown, contact the supplier. Most companies that make commercial site furniture will do custom powder coating or custom slat colors for an additional charge and a longer lead time.
A few things to keep in mind. Contrast usually looks better than colors that are too close to each other. A dark frame with lighter slats gives the bench more depth. If the bench will sit in direct sun, remember that darker slats get hotter, so lighter colors may be more comfortable. Also ask whether the slat finish is UV-stable and whether the frame powder coat is rated for outdoor exposure. For wood slats, you may be choosing between natural, stained, or painted finishes, and painted wood will need more maintenance than recycled plastic or metal.
So the short answer is yes. You can get a street bench with a different color slats than the frame. Just be ready to specify both finishes when you order, and if you want something unusual, ask about custom color options before assuming the standard catalog is your only choice.