Here is a great example of underscoring with a bit of black.
Look at that wonderful yellow backsplash. It's just the right amount of bright color, and the bright colored chairs mix right in with it perfectly.
Sharon Portnoy is a prescribing just the right doses of blue, yellow and red.
Adding apples and lemons is one of the best ways to add punches of red and yellow.
As are flowers.
And Tonka Trucks!
The primary colors are a great starting point - you can slide into different shades from there.