What does "interpolate no end color found" mean ?"

[micheus]

I think one needs be positioned in-between on an empty cell ?

yeah. The information line shows "Interpolate empty colors" when the item selected in the context menu is "Interpolate" and this option just appears in the context menu if you activate it in a empty color slot.
It's ok, the info for menu items change so fast that sometimes we just don't remember to check it. I already did that.

I'm all good with this now.

Nice.