To find surface defects: Tamiya sky gray XF-19 (acrylic).
Under white: any metallic silver.
Under yellow: white, or silver then white.
Under orange: same as yellow unless you want a slightly darker shade, in which case use a brown.
Under red: same as yellow unless you want a slightly darker shade, in which case use a brown.
The key to using acrylics successfully is to have a CLEAN plastic surface. I suggest washing using Simple Green or Windex, rinsing thoroughly with distilled or deionized water, then wiping down with ethyl, denatured, or 90% isopropyl alcohol just before painting. Once you start painting, don't touch the model with your bare hands.