Depending on the detail of the tire. Some have recesed treads and others do not.
Usually with the recessed treads I will paint the sidewalls a flat black or scale black and the tops a lighter gray ( either gunship gray or a darker gray).
Then thin a the same color down as a I painted the sidewalls. Apply it to the part of the tire with the tread on it. Gently wipe it off only leaving the darker color behind that went in the recesses of the treads only.
This gives the tire a used look of being rolled down the tarmac and runway and the abuse they take on landings.