Re: Paper similar to newsprint that won't yellow?
Welcome! There have been discussions here before about what to do with newspaper. One thought is that if it is sealed well with an acrylic or resin layer, it won't yellow (at least it won't yellow as much). We don't know for sure that's true though. I tend to be cautious and make photocopies of the newspaper that I want to use on really thin copy paper. For you, though, this wouldn't have anywhere near the same texture, so I don't know that I'd recommend it.
Do you think you could add an isolation layer over your watercolors to prevent running and then seal it? There are some good sealants that have UV protection built in. Golden's makes some, as an example.