It's so easy...

...I could even do it in my sleep!

Take any support (in this case, a well-used mat board 'tester'),

and sponge it with a paint-covered paper towel, sponge, plastic wrap, cloth, etc.  Here I've used a paper towel to add a turquoise to the mix.

Repeat the process with another colour (I've used  lavender here),

and another (greyed lavender),

 and another (purple).

You can continue the process until you're blue in the face (or purple, turquoise, lavender, grey, etc.) and you've got a wonderfully subtle background for your artwork.