I've not come across anyone applying a mirror tint to amber lights yet, so I thought I'd try it. This a 50% mirror tint film. I think it gives the turn signal a slightly translucent look and tones down the orange.
I'm tempted to try a 20% mirror film (i.e. 20% light transmission rather than 50%). The 50% barely reduces the light output even in daylight, so I reckon 20% will still be OK.
Before:
The tint film only cost £5.99 per metre (50cm width) from eBay.