If someone does not have water-color paper, what can they use instead as a canvas or as a paper substitute?
For example, would a white-coloured cotton T-shirt work well?
Would white denim (like you use for making pants) work well?
Can you apply water-colour paint and/or dyes to fabric as if you are painting on paper?
You can apply paints on fabric, but I would never call fabric a "substitute" for any paper because the paint behaves so different that you actually need different skills to produce similar results.
Water-colour paper is a very dense paper. The fibers are compacted and at least one surface is smoothed so that water cannot penetrate deeply and dissolve the paper. If paint is applied onto dry water-colour paper, it stays in place and doesn't bleed. If you want bleeding colours, you have to add water to the paper before applying the paints. As an artist, you can control how sharp or blurred the lines become that way.
Fabric OTOH is much looser compared to paper and any water you add will immediately soak through the entire thickness and spread out. Since water-colours do contain a considerable amount of water, they will soak and bleed just as much when painted on untreated fabric. The pigments may just sink to the bottom (= backside) of the fabric, or the dried paint may just flake off, leaving you with a very faint image.
Art canvas is technically also fabric, but it was treated with a primer - traditionally gesso - which fundamentally changes its properties and makes it behave more like paper than fabric. The texture of canvas means that watery paints will sink into the tiny dibs and create a mottled structure which is usually not desired in water-colour paintings, but may be something to experiment with. To hide this texture, artists traditionally use more viscous paints on canvas, which in turn require different brushes and techniques than water-colours.
In my personal opinion, the only substitute for "real" water-colour paper is very dense paper or cardboard. The thickness isn't very important, but how much the fibers have been compressed during manufacturing influences how quickly water can soak into the paper and dissolve it.
