Hi all - trying to use an old densitometer to read my delta 100 negs, and not sure which colour to use. It has a dial on the front with four colours, red, blue, green and yellow, but no 'visual' cahnnel. I'm guessing yellow might be the visual channel, but only guessing. Any thoughts? (it's an old orange unit, Rollie 401T, or Rollmat or something like that).