TL, we do have the ability to make a setting in rose gold. It typically costs a bit more because the metal mix is different and there aren''t a ton of sources.
Whether or not it would be custom really depends on how you wish to do the setting. For example, in KT''s scattered rose gold band, she didn''t change the design at all so it was semi-custom as opposed to custom. Custom pieces aren''t refundable/returnable, but semi-custom pieces can be returned less the restocking fee.
With respect to selling mountings only, we do so for most of our settings. If the setting is a "Whiteflash exclusive", the website usually denotes that the setting won''t be shipped without a diamond set into it.