Business casual means no jeans, right? Can you wear skirts or dresses?

No denim of any kind. And, the registers face the front door where it can get cold.

You might try microcheck pants with a cowl neck knit top or buttondown shirt. A pair in wool would be warm. Brown or grey pants would look good with black too.

I would try to stock up on colorful knits and sweaters.
Slacks - microchecks or tweeds are a nice variation on solid colors.
Can you wear any color top?
Wish my company would do the apron instead of our company shirts and vests - they get so boring and honestly don't look lthat great.

How about knee-high boots under dress pants (for more warmth), since denim skinnies and boots aren't in your dress code.

Thanks for the great tips! Today I bought:
two turtleneck sweaters, teal and black.
a funky fly away rust cardigan without bulk/draping in front (because it looks weird under an apron)
a teal fitted buttondown shirt
a dark red tunic sweater.
I can layer camisoles and silk long underwear underneath for warmth and everything will look good with my brown and black straight leg pants. I still need to check my short necklaces since they are the only viable accessory with the apron. Any further advice is welcomed!

Since it seems that your tops will be covered with the apron how about 3/4 sleeve sweaters/tops/shirts and some statement bracelets?