Is it possible to SLI two different GTX 970 graphics cards?

Solution
It will work, but **at best** you will run at the slower ones stock settings.... OCing will be very iffy and that's one of the big advantages of the 9xx series.
Yes, the faster of the two cards will be slowed to match the slowest card in the setup however.

Just a personal side note here, I would be bothered much more by two different cards than I would be some colors not matching. The reference model and the MSI TFV cards look very different in a system.