No spoilers (may as well make a fool of myself in the open). I was looking for a 6-letter shade of green that started and ended with the same letter. Discarded WILLOW because of the double LL.
Tried LAUREL and actually thought I was onto something when I substituted 28,31,48,21,18,28 with line/letter from Green Lantern Oath and got H,L,Y,N,T,H. 3 of the letters - L,R,L are +3 in the alphabet, U is -3. but the other 2 just don't fit. I bet you all thought of that anyway.
As for the paint brushes and the letters on the cartoons (I admit to knowing precisely 0 about GL so I had to Google): Kyle Rayner is an artist IRL. Could the H and K be initials of past or present names of GLC? And it seems the duo are mirror images of each other: it wouldn't take much imagination to mirror image K as Lambda (from Ep 1).
Like I said, not much. But there if you hadn't already thought of it and it's of any use to you.