I absolutely love Holbien Acrylic Colored Gesso. Gesso is primer for your canvas or pretty much any other surface (wood, glass, fabric, robot) you want to paint on) and I love to start with a bright bold color and there simply are no brighter colors than Holbien.
I pour it right into the canvas and then spread it with either a discarded card if I want texture of a big house painting paintbrush if I want it smoother, don’t forget to paint the sides.
Holbien Acrylic works great under both oils or Acrylic paint or even pastels. Keep your paint brush mostly dry as it isn’t as nice thinned out with water.
Holbien comes with a heafty price tag $27-$70 depending on the color, so here are some alternatives that are maybe almost as good. You can mix all your left over canvas on your pallet with some white, you will probably get a lovely taupe, cover your next canvas with that. Or you can use acrylic paint, unless you are using a cheap paint this isn’t really a saving though and you can’t oil paint over regular acrylic paint, but you can acrylic over it, this is a good use for that acrylic tube of a color you bought two years ago in a size way too big in a color you just don’t use. Happy painting.