I know you cant have vaginal sex before a colposcopy but im wondering if anal sex is safe a day or so before the biopsy?

You can if you must

Anal sex would not interfere with a colonoscopy with and/or without biopsy. Following the biopsy follow strictly recommendations from the team including when able to resume sexual activity.

While anal sex would not interfere with colposcopy, it is not such a good idea, Anus and rectum are designed for sex and you risk developing anal incontinence from anal intercourse.