Light or dark colors aren't the problem with this product. Durability is. Do a search on FMJ durability on this forum and you'll see that it is poor. If you really want an awesome sealant then go with Zaino. Especially on light colored cars. I used FMJ exclusively for two years so I speak from experience. Then I found this site and tried other products that blew FMJ away. I did some comparison tests between FMJ and Zaino just to be sure. After the test, I couldn't return my FMJ fast enough. I Zaino'd my Tundra 6 months ago with Z-5. It remains outside 24/7. It has only been washed 5 times in six months. However, each time I wash it, it beads and repels water like it did the day I applied it. The FMJ stopped beading the day after I applied it. There's a reason that hundreds and hundreds of members of this site rave about Zaino. Try it for yourself and you'll see why.
I haven't had the durability issues with FMJ that Patrick apparently had. It works well for me in that it continues to bead water after 3 months. I usually "refresh" the FMJ after a wash with the Menzerna HGAS; I do a QD wipe-down with the HGAS and the car looks new again.
Re colors: I have used FMJ on my black BMW and also on my wife's bright Caribean Blue G35. It looks good on both to my eyes.
Not really sure why FMJ gets a bad rap. Really good product, looks and durability. I have right now 2 coats FMJ and one coat AW. Beads like crazy, slick as glass, and is almost self cleaning. Quite amazing, actually.