It'd probably cheaper to keep the WT/ET boarding pass white (as per the current ones).

Of course there will be a bit of an issue with the SSCI equipment to issue coloured boarding passes because I don't think they are equipped for colour printing or having a number of paper stock.

LH etc have the class of travel in a big letter (but it's in a dotted print on a printed ticket stock so it doesn't make it stand out). If it's printed on a plain stock, it which might be a low-cost alternative to coloured boarding passes as it doesn't need to be stared at to identify the class of travel. Swiss one is a little clearer as it is printed clearly on the stub.

Also perhaps F boarding passes issued by the check-in desk could have the edge in a different colour.

QF used to issue different-coloured sleeves depending on the class of travel. I am not sure if they are still doing them as I have not been given a sleeve for a while. However, I think anyone using the F check-in desk ended up with the gold-coloured sleeves for F.

CX stub has 3 different coloured blocks for the class of service onto which the seat number is printed (not sure what they do with a 4-class serve though).

With mobile BP it's easy - different-coloured background

P.S. I think F should be fluorescent pink

Purple airway as well.