I’ve had leftover sheets from a variety of sources, including reused packaging materials, but the colors often get muted or the paper falls apart under the acrylic and water.
So I splurged on some professional paper and got excellent color retention and absorption.
If you look hard enough and want to spend a fortune, you can get different paperboard thicknesses and colors.
While not my favorite material to work on, it worked very well with the dry pastels I was using. The smooth surface held the color nicely, but was very easy to smudge, even after a coat of spray on varnish.
It warped a little under a collage treatment, but the Mod Podge held well.