This is what I do with my old watercolour paintings that go wrong or I no longer want .
In this case I made my own pastel grounds with 3 parts black gesso to 1 part fine pumice powder mixed together thoroughly.
Tape the paper to a board with masking tape with a border of about 10 mm. Once you've taped the paper down just paint on the mixture so it completely covers the paper. At this stage the paper will probably cockle.
I then dry the paint with a hair drier untill it's dry to the touch. At this stage the paint will form a skin holding in the moisture between the paper and the board. Just remove the paper from the board and turn it over. Re-tape the paper to the board surface down and dry out the under side with the hair drier or leave over night. Remove the paper from your board and place under something flat and heavy to stop the paper from warping. That's all there is to it. Just pack the paper in a plastic sleeve the use it as you need it.
For other colours just use white gesso tinted with a little coloured acrylic paint.