Q.My Salix Integra or flamingo Willow has completely changed character, lost the characteristic variegated leaves with cream and
pink, and the leaves coming out are now just plain green. It was in a pot and I have had it for over five years. Last year we were away on holiday and found on our return all leaves had browned. I cut back leaves — perhaps timing was not right. What have I done wrong, and does it mean it will never go back to its original form? I intend buying another one and would like to know to avoid this recurring.

It sounds like your plant may have died off and that the root stock took over.
Inspect the trunk and see if the new growth is coming from below the graft line.
If you still have growth from above the graft line you can try removing all the root stock growth.
https://www.gardeningknowhow.com/ornamental/trees/willow/dappled-japanese-willow.htm