A little while back, I discovered the most gorgeous stuff by accident, as I went out without my watercolours, but did have this little bottle in my kit. I had had it for ages, and never used it, not even sure why I bought it. In fact, it was enormous fun, so I eventually did a whole sketchbook - a spiral bound pad of watercolour paper, actually, so that one could then tear out individual sheets and frame if a masterpiece was achieved (! well, gotta live in hope), if not, then at least they stayed clean and safe. No need to stretch the paper using this method. The stuff I used is called
DR. PH MARTINS RADIANT WATERCOLOUR, and comes in bottles with a rubber eyedropper thingy at the top. You can use them direct from the bottle, or can dilute with water.
Here is a pic of the bottle: