Many models look good with more than one colour on. Most models are easier to paint in parts. To aid this, don’t assemble everything before painting. Take some time to work out which bits need to be one colour, and which need to be another, and keep them separate. They may even need priming a different colour (all my Eldar vehicles for example have a light underside requring a white primer, and a dark top requiring a black primer).
Illustrations of this rule will happen throughout this project, as the Necrons are painted almost entirely in pieces. In fact on the Warriors no parts were glued together before painting began. Time will tell how wise or foolish this decision is…