Distinguishing Buff-tailed and White-tailed bb s is always a challenge! Often its not possible particularly the workers. Also its not always possible to make accurate identification from photos alone.
This one to me looks like a White-tailed as the tail is white and the yellow band on the collar behind the head looks bright yellow. The yellow in Buff-tails is always dark/ golden.
Not 100%sure, sorry.
Thanks very much Elaine, I do appreciate your help.
It is an exquisite image Richard