The two colors are not even close. Mystic Blue is a lot darker than Topaz Blue. Out of the two colors Mystic would be a lot closer to the color of your car, but even then it isn't even close.
Topaz Blue turns a lighter shade when the sun hits it and darkens up at night or when cloudy. I really really love Topaz blue. It was one of the colors I was looking for when I bought my car. I haven't seen a lot of Topaz Blue on M's around where I live. I will post some pictures (when I get a chance) of the Topaz Blue when it is in the sun and some when it is darker.
Here a few that I took last year in the bright sun. There is a lot of glare on these, but they came out decent...
Topaz Blue