There is a lot of mis-information about undercoating Alclad....
For most of the colors, you do not necessarily have to use an undercoat, although a good, smooth one doesn't hurt. The stuff is a lacquer and can craze bare plastic, even when applied with an airbrush. And, an undercoat will help if applied over putty, etc. It should be smooth but does not have to be a particular color. Tamiya's Extra Fine spray primer works well.
For the high polished/bright shades like Chrome and Polished Aluminum, you should use a gloss primer. Black does work, but it doesn't have to be black. You can also influence the final color a bit by using Red, Blue, etc. as a base. Alclad makes their own primer which I have had mixed results with (the stuff has a habit of never drying). But, Tamiya's spray lacquers are great either right from the can, or decanted and airbrushed. Tamiya gloss black acrylic is also a good base. You can use enamels but they should be fully cured before applying the light mist coats of Alclad lacquer on top, and the end result may be a bit duller than using a more durable base.
Alclad also makes a number of non metallic paints so you obviously don't need to follow the same base coating instructions. They offer stuff like "armored glass tint" that you apply straight to your kit's regular clear plastic parts. And the candy car body colors go over a metallic base coat of your choice like gold, silver copper, etc.