The original poster : I planted blue Delphiniums and they grew tall
(about a metre) and strong. At the end of the Summer (1998) I cut them
back to ground level and left them in. The next Spring (and every year
since) they have come up white; equally vigorous in growth and with a
storm of blossom on them.
I wonder if I could influence them in some way to change colour again
?

Are you absolutely sure what you have had from the second year on is
the same plant and not progeny of your original blue plant(s)? Our
local wild Delphinium species comes in both colors, with whitish being
far more common.

M. Reed