Thanks Michael 1974. That was in fact what I had presumed to be AgX's point and he confirmed this but always nice to get confirmation from another user
MartinP Thanks for your reply. Most easels like the Paterson, the Durst Comask etc seem to be white and yet I haven't noticed a problem of light spillage with a neg in the neg carrier but a dark red paper might help and I understand why it has advantages over a black paper. However by itself a red paper on the base board doesn't for instance substitute for a safety filter which can be swung across when you need to get a dodger in position or need to place a cut-out in position for more complicated dodging
I now understand, I think, the function of your red card but on balance, I still think that a functioning red safety filter under the enlarger lens has advantages.