You cannot put a custom color into an official part, no. You're free to put a custom color definition (0 !COLOUR) into an unofficial part, and then use that colour definition inside the part. You'd want to be very careful about the color code you chose, though. LDView improperly makes all custom color definitions global to the entire model. This isn't correct behavior, and may be fixed at some point in the future, but that's the way LDView behaves right now. Additionally, the only finish flags that LDView recognizes are RUBBER, CHROME, and METAL, and what it does with those is frankly somewhat lacking.
If you feel that a new custom color definition is needed in order to model an official part, you can request that it be added to LDConfig.ldr. Once that has been done, you can then use the new color code in an official part.
So unfortunately, I think the basic answer to your question right now is, mostly, no.
If you feel that a new custom color definition is needed in order to model an official part, you can request that it be added to LDConfig.ldr. Once that has been done, you can then use the new color code in an official part.
So unfortunately, I think the basic answer to your question right now is, mostly, no.