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 ?

I would guess that you had two seedlings stuck together. When you cut them back
to ground level (which you shouldn't have done; you should have left the basal
leaves), the blue one probably died & a white one survived. If you want blue
you will have to plant another one.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)