I know this is odd advice coming from me - the vivid and colorful painter that I am - but grey is a great color to offset those vivid reds, blues, and yellows! But how, HOW, you may ask, do you get a good grey? Black and White? Boring!
The color wheel is not only a great place to learn how to make colors POP (placing compliments such as red next to green, for example), but it is also a great place to learn how to mix a great grey color! Try to mix your red and green together, or blue and orange, yellow and purple... see what great and varied grey colors you can get! Of course, you can always go with the tried and true... SLUDGE. That's what a messy painter's paint palette (that's me!) turns into after a long paint session - ALSO a great grey!
Have too much grey? Use it as your base coat for your next work of art - as shown on my latest painting here!
"Pear Tree and Partridge"