To add, Hua Jian Min teapots uses different base clays (which I presume is the clay seen on the inside of the pot) . The clay can looked yelowish, pinkish, or greyish.
The outer surface (after yao bian) of the pot can look golden yellow, reddish-grey or even burnt grey. Some pots can have one shade on one side and assume another shade on the other side.
This is what I gather from casual conversations: The popularity of Yao Bian is relatively recent, much like abstract art. In the past, Yao Bian on a pot would imply an improperly fired one and the pot would likely be discarded.
Seems like there are many variables and complexities in Yao Bian.
Is there any particular colour that collectors usually look for?
Any other makers that I should look into?