I’m unfamiliar with the terminology you use, and can’t seem to find it on the Internet. The white powder might be white chalk used to make the watercolors opaque. If that is your aim, a tube of any white waterbased paint will probably do. You can also probably get the same results using gouache paint, Winsor Newton being the best.
Your best bet is to contact a miniaturist society. There are many on the web.