Thanks for the helpful suggestions Elaine! I will look into them and see where it gets me - I have a feeling I have tried the perframe/scolor idea in the past without success but I may be misremembering.
One other thing - I'd like a one command way to convert a full atom model to a Calpha model, keeping the helix/sheet records at the start intact. This would be very helpful when running morphs of a large protein (where often I do not need to display sidechains, just ribbon), because it makes the calculations much faster (e.g. rather than 120,000 atoms I have ~17000 using CAs only). Currently what I do is I delete all the non-CA atoms in chimera, save the pdb, remove all the TER flags chimera has added between each CA atom using grep, and then replace the HELIX/SHEET records with those from the original file (because they get corrupted for some reason in the process. This works fine but it would be nice to have a one liner in chimera to handle the same at some point (although I realize it is a pretty niche functionality and maybe not worth it). (alternatively a "coarse grained" morph where parameters for each residue only take into account the CA would accomplish the same thing I guess.)
Cheers,
Oli