Hello,
It is very easy, just use the color names you want in commands something like the following:

color coil gray
color helix orange red
color strand purple

... the above would color all the representations (cartoon, atoms, molecular surface...).  If you specifically want to color cartoon only (not atoms, molecular surface...), could be something like:

color coil gray target c
color helix orange red target c
color strand purple target c

explanation of command-line specifiers like helix, strand, coil...
<https://rbvi.ucsf.edu/chimerax/docs/user/commands/atomspec.html#builtin>

list of color names
<https://rbvi.ucsf.edu/chimerax/docs/user/commands/colornames.html>

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Jan 6, 2023, at 8:11 AM, Enrico Martinez via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:
>
> Dear ChimeraX users!
> I need to color cartoon representation of a protein according to its
> secondary structure (e.g. to color helix in red, b-stands in blue and
> coils in violet etc). I have already seen a built-in plugin in UCSF
> Chimera (but not in Chimera-X) that could do this job! Could you
> suggest commands that would be useful in Chimera-X?
> Many thanks in advance
> Cheers
> Enrico