If I remember correctly (and I may not) I think I saw the original art to the GL cover and it was done in wash. Then color was added. I am not familiar with this particular Adam Strange cover but it looks the same. They seem a bit darker from the GCD photograph that I remember the original GL cover being.

At the time, I think Russ Heath used a similar technique on some Sea Devils covers. Once in awhile this techinique would be seen on covers here and there in the silver age. Somebody somewhere probably has an index to all of them...