Hello!
This happened quite a long time ago, about 6 or 7 years ago.
The sun had just set and it was quite cloudy and what me and my friend saw was the clouds changing colours in a strange way.
One part of the sky would change to a purple colour, slowly fading in and then it would fade out and then a different part of the sky would turn a green colour and faint red colours, by the time we went in, there must have been four or five different colours over the sky, going for a good few miles (can't remember very well, but possibly 5-10km), although we couldn't see behind us as a building was blocking our sight.
The diameter of the blotches of colour must have been about 1/2 - 1km but very high up, although I can't be certain.
I've always wondered what it could have been. I'm very sure it wasn't some kind of spot light, as there was no beam coming from the ground.
Unfortunately I don't have any images or video.
Thanks.