What does it mean to have a four-color cover?
Bookmobile quotes spec a four-color cover, but the four is the process of how we print the cover—you are not limited to four colors on your cover.
We print color with a four-color process, also known as CMYK (cyan, magenta, yellow, and black). So those four colors are mixed to create an almost infinite rainbow of colors.
We cannot print white, neon colors, or metallics (though we can foil stamp a metallic on a cover).