If you want heat/UV protection in the rears, I would suggest putting 70% over the pre-colored glass. The factory glass is around 24% and so the 70% (which is typically the VLT of "clear" factory glass) will drop it to around 22% ish depending on the brand you use. In that case, putting 20% on the fronts will probably get you to the closest match when all is said and done. Most manufacturers will only offer a 20% or a 35% and nothing in between. 35% will be noticeably lighter and 15% (next level down from 20%) will be too dark. You might get lucky and find 25% which will match the stock rears but if you put any sort of heat/UV protection on the rears, it will end up just a smidge too light.
If you don't touch the rears at all, I'd go with the 35% since the factory ~70% VLT on the fronts will net you around 25% which should be very similar to the factory rears as
@OCJeff says.
TLDR: If you plan to add heat/UV protection all around, go 20% in the front and 70% for the rears for the closest match. If you do not plan to touch the rears at all, go with 35% in the front (or 25% if the manufacturer offers it).