My Paolo Soprani CBA (C system ) has all white keys.....doesn't matter what colour they are as I can't see them!.......I was advised to mark the C F and G keys with something I could feel - as a guide. I used some rough textured plastic discs that I stamped from a sheet and stuck them on with fine quality double sided tape - not at that stage wanting anything that was not reversible. After about 8 months, a couple of the discs are starting to move and I'm thinking of a more permanent solution. Has anybody got any suggestions re the best markers to use, where on the key to place it and whether or not something glued into a newly drilled hole would be best. Thanks in advance for your comments.