The label side can be silver or white, with some companies printing custom labels.
Usually CDR promos do not have a catalogue number, and often contain unique mixes which don't get commercially released.
The info. on these can either be printed on the disc, these are the ones that go out to radio stations etc., there are also custom pressings, these are usually handwritten, and produced in much smaller numbers - and some are the cd equivalent of the acetates of records, i.e. a test pressing.

I have quite a large collection of these going back to the mid 1990's - some are probably one-offs?

Please be aware that these type are extremely easy to fake, so care should be taken if you are paying a high price for something apparently rare.