sed -i -e 's/tp-link:blue/tp-link:green/' /etc/config/system
As @slh points out below, the "tp-link:blue" reference in ar71xx was from older units. It's not that ath79 magically changed the colors of the LEDs, but changed the device and UCI labels to agree with many of the Archer C7 v2 production units.
I assume the situation with the archer c7 v2 is similar compared to the one with the tl-wdr4300 before, rev 1.0 and 1.1 of the tl-wdr4300 shipped with blue LEDs, somewhere between those and rev 1.5 they switched production to (cheaper(?)/ longer lasting(? the blue LEDs do visibly age) green LEDs.
Edit: If you look at very early reviews of the archer c7 v2, you will notice blue LEDs, while more recent pictures show green LEDs, corroborating my hunch.