Most tanks of this sort in America are typically painted white or silver to counteract the effects of solar heating upon the tank. I choose to assemble the sphere of the tank, add the legs and ladders, but do not glue the legs of the tank to its base. The legs can be temporarily inserted into the base while the glue is drying; once the glue has set, the assembled tank can be removed, spray painted in the color of your choice, then glued to the base for final placement on your railroad. The kit comes with graphics for several oil companies, and you can also make your own graphics with a computer onto paper or decal film. The Faller graphics are self-adhesive paper, but you can place your custom graphics printed on paper with a thin film of glue.
In the background of the photo below is a Faller gas tank painted white, adjacent to the fuel depot.