Some Holmegaard colours seemed to be pretty susceptible to gradation - blue, mostly. Reds and yellows always seem to be consistantly coloured, though.
But you're right, it's not a HG piece. I'm sure I've seen this vase before, though - I just can't remember where. Sorry.
But there have been a few threads about bamboo vases on here in the past - see if a search yields anything.