Colors don't exist alone. There is no white on white. If is then it's just simple full white.
Once a color is paired with another color they differ; they become engaged in a relationship. They interact.
Their interaction is multiple but most importantly is about contrast / hierarchy. Which is the dominating color bringing forward it's content? Or a dominated color bringing into background it's content?
Hierarchy exists on a monochromatic composition too; we don't have to add vivid colors to differentiate. Vivid colors are—only and only—to bring in the mood.
Source: It's Only Color