I use watercolor as ink on perfectly normal envelopes. It's a lot of fun because the colors can be very bright, and you can also change colors as you go to introduce a gradient.
I love doing the orange to yellow gradient and the blue to green and the purple to red...the rest of the gradients I try tend to get muddy.
I use my tiny Windsor & Newton Sketchers Pocket box, and that works really well (love that thing!). Santa brought me a Kuretake Gansai Tambi set that I absolutely cannot wait to try! But...as far as I'm concerned...all watercolors are great for using as ink! As long as you get the water to color ratio correct.
Glad you've discovered this! So much fun.