Alexandre Korney wrote:I had a question concerning last layer of plaster on Rocket mass heater: I would like to use pigments in clay plaster or lime plaster but I have no experience in this, is there anything to know or any specific documentation that could help me to finish our dragon... 
A simple and straight forward documentation that has worked for me:
clay plaster recipe
As for adding colours, I have found that for the best vibrancy, you can't beat the cement pigments. Not a lot is needed and there are quite a few colours to choose from.
I have tried food dyes before and found the colours to be really unnatural and gaudy looking.
Sorry, no experience with lime yet.
I've tried some crushed charcoal in my last batch. It turned out to be quite chalky (without wheat paste) and the colour turned out greyish blue.