a few things...
safety: safety first and last
- dont use glass, if shattered you will loose your eye too easily
- use quality only, if not your eyes/retina will get scorched by the sun without you realising it untill its too late
- never use red (or green) in traffic, obvious, but also not in direct sunlight, as they r unable to filter out the harmfull rays
low light: yellow secondly amber to brown, yellow gives the most contrast, brown the least, as a pro driver i use yellow in fog/snow, i see more/further, with more ease
direct sunlight on the flats; good dark shades, too keep out enough light
good luck on your choice