Red: Str (obviously)
Yellow: Str/Res OR Str/Haste OR Str/Mastery..this color entirely depends on how you want to play imo. Since resil gems were halved, I find having a pure resil gem to be hurting your own damage since all have 65% base now. I am also considering possibly pure mastery gems for yellow or haste, thus enabling reforging to expertise and hit more efficiently. Again this is all speculation, not saying what is going to be better.
Blue: This is a bit difficult to pin which gems would be good here as any other pure blue gems are pve oriented or non DK, and most of the hybrid greens and purples give DK's 1 priority stat with a non priority. I.E STR/STAM or HASTE/STAM.
PVP power seems to be pretty much worthless for us to gem at the moment. I will be doing some more testing here in a bit with my current gem setup and then update this post to inform those looking to continue PVPing Frost.
And yes I know I don't have any arena on this toon, nor much PVP in general. Just attempting to help those who don't have the time to figure this out.
Just did a BG and wow, just wow.
Highest Obliterate crit was 45k, Frost Strike highest crit was 19k and HB is laughable at best.
So yeah it is official that all resil and pvp power gemming is worthless.
I'm still not sure what to gem in blue sockets but we shall see.