".........much to my surprise the leaves aren't especially dark. Indeed no
darker than some others we have that are green leaved.
Perhaps it's because it's still in the greenhouse and it will darken when
planted outside............"

You could try giving it/them a light feed of Miracle grow, might help to
bring some depth of colour to the plant/s.
Being inside should make no difference, unless possibly you have it at a
high temp, but over the last few weeks that's not very likely, with some a
lower temp. brings out better leaf colour.

Can I ask where you got the plants/tubers from?
If Dutch tubers then there is a chance of a few wrong ones getting into the
batch, They produce them by the 10's of thousands, and keep them trimmed so
that they don't flower, as this would weaken the tubers. With production on
this scale there are always some that are better than others.
When it was first bread by Tresseder in Cardiff, it was propagated only by
cuttings, so the best stock was always selected as stock, now with a lot
being produced by Tissue culture the selection isn't so critical.