Tysrett, I have been giving this a lot of thought..when I had the time between other things...and I have found it almost impossible to achieve something decent on the budget you have.
HOWEVER, on my way back from Dartford tonight I had a bit of inspiration..
Why not buy three 70W Son floodlights, mount them at ground level, fed in SWA. If you put one in the middle of the three exposed sides at ground level pointing up at an angle toward the middle of the pergola, and cover each with a coloured filter, the effect could be quite nice.
If you were to choose Red, Green and Yellow filters, you would get a pall of white light in the middle, but coloured light on the sides.
The filters may be the killer here, as to get the effect right they would need to be optical colours, not any old red,green,yellow coloured glass or perspex, and they may break your budget.
But, I suppose you could just use blue or green coloured glass in front of the floods to give a nice effect.?