You should be able to have the big box store custom mix paint colors for you, When I painted the old garage last summer I took a scrap of vynal siding from the house and had the paint pro put it in the spectrograph to read the color and tint formula and then had the primer, topcoat and rustoleum for the steel door all tinted to match. The primer and rustoleum weren't a perfect color match but they weren't the same white as the tintbase topcoat that the color was formulated to work with.
If you decide to custom tint your paint make sure that all of your white gallons are from the same batch number and that you get enough paint the first time, and/or mix the gallons together to assure color consistency.
Warning: once you use industrial type paints, automotive paints usually don,t adhere properly untill all residue is removed