There are several ways to go about it, none of them are wrong. What really matters is what you want to convey as the "theme" for your army.
As an Example, my DE army is a desert theme army, I have lots of reddish golds( not metallics) in my army with bone white. The overall tone of my army is reddish gold or like sand at sunset.
The color I have selected for my base, wanting to keep the desert theme, I used a greenish brown as my base color, then highlight up like I would for a desert with more beige sand colors. Since red and green are contrast colors to one another, the model and base stand out fairly well against one another even though they are desert themed and have the same mid tone colors, that being the gold browns.
With my Metallic Silver Skull space marines, I did very earthy browns and greens to contrast the metallics of my marines.
Hope this helps.
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