This was the third of three arch clouds, or mist rainbows, or whatever they were we saw today on An Riabhachan. We'd seen a Brocken Spectre as the sun tried to get through and then these appeared. Perfect arches of cloud or mist, and each end had a slight colour to it, like a faint Brocken Spectre halo. The photo doesn't do it justice but I've never seen anything like it before in my life. Any thoughts/ explanations?

It's a fogbow, see http://www.atoptics.co.uk/droplets/fogbow.htm for an explanation.
It's formed in the same way as a rainbow but because the water droplets are a lot smaller it appears white instead of rainbow coloured.