Depending on the site, you may be able to submit both color and bw versions. If that's not allowed, I would default to color because, as mentioned, just bw can limit the marketability of the image.

I've sold Santa's in January, but, I agree with others here that you can usually see an uptick in Christmas imagery around the end of summer (northern hemi).

Link to my Behance port with 30 or so cover images (books, cds, etc.):

http://bit.ly/1ceqEKI

Most recently, one of my images was used on a re-release of Herman Wouk's classic "Don't Stop the Carnival"

On Behance? Leave me a comment and/or like and I'll be sure to return the favor!

Link to my Behance port with 80 tear sheet hero images:

http://bit.ly/KDcF3r

includes appearances with Ritz-Carlton, Travel +Liesure, Marriott, American Express Vacations, Travel Channel, Forbes, British Airways, MSN Luxury Review, Diamond Cruises, Staybridge Suites, FoxNews.com, International Living, USA Today Travel...

On Behance? Leave me a comment and/or a like at the bottom of the Gallery and I'll be sure to return the favor!