I don't have a current Tumbleweed installed
but it works for me with openSUSE Leap 15.5
with a test print queue 'RicohAficioSPC420DN'
(I don't have that printer model)
using
/usr/share/cups/model/OpenPrintingPPDs/postscript/Ricoh-Aficio_SP_C420DN.Postscript-Ricoh.ppd.gz

It works for me according to what is described in
https://en.opensuse.org/YaST_Printer#The_Modify_Dialog

I.e. the YaST2 printer Modify dialog
via [All Options for the Current Driver]
shows a new "Driver Options for Queue ..." dialog
that lists all options for the current driver
where one of those options shows the current state
ColorModel/Color Mode: CMYK
or
ColorModel/Color Mode: Gray
and clicking on it shows the available values (CMYK or Gray)
where one can see the currently saved value
and one can select an intended new value.

I verified via command line that changing the 
ColorModel/Color Mode: CMYK
to
ColorModel/Color Mode: Gray
and vice versa
results that this driver option actually changed
(lpoptions shows the current setting with an asterisk '*'):
----------------------------------------------------------------
[default after print queue setup with YaST2 printer:]

# lpoptions -d RicohAficioSPC420DN -l | grep ColorModel
ColorModel/Color Mode: *CMYK Gray

[then changing in YaST2 printer from CMYK to Gray:]

# lpoptions -d RicohAficioSPC420DN -l | grep ColorModel
ColorModel/Color Mode: CMYK *Gray

[then changing in YaST2 printer from Gray back to CMYK:]

# lpoptions -d RicohAficioSPC420DN -l | grep ColorModel
ColorModel/Color Mode: *CMYK Gray
----------------------------------------------------------------

It is a different things whether or not
the ColorModel option setting enforces the printer
to print everything in CMYK mode or in Gray mode.

Because it is a PostScript printer it could happen
that the PostScript that is sent to the printer
contains PostScript code snippets that specify
CMYK mode or Gray mode for the particular printout.
In this case a PostScript printer prints according
to the setting in its current PostScript data
regardless what the general ColorModel setting is.
In this case it would help to specify color or
monochrome prining output in the individual
application program wherefrom one prints.

FYI:
You may also have alook at the section
"Printer drivers: Make the printer print" in
https://en.opensuse.org/Concepts_printing
therein in particular
----------------------------------------------------------------
Usually there is neither such a thing as
"the one and only driver" nor is there such a thing
as "the best driver" for a particular printer model.
The better the printer hardware is, the more different
drivers are available, where each driver is best
only for a particular use case.

For example a PostScript+PCL color laser printer
could be operated as

* PostScript printer using a PostScript PPD
* color PCL printer using a PPD for a color PCL driver
* monochrome PCL printer using a PPD for a monochrome PCL driver

Printing via a PCL driver is usually faster
but with less quality than printing in PostScript mode
and using a monochrome PCL driver is probably the best way
to enforce fast monochrome-only printing. 
----------------------------------------------------------------