How can I bathe with a colostomy?

As usual. Much like bathing a toddler being potty trained - accidents will happen but life goes on.
Shower and reapply. If the stoma is mature you can bathe normally, but you will need to reapply the pouch after your shower. Talk to your ostomy nurse and see if she has any suggestions or limitations for your particular ostomy.
Easy. Okay to shower with all parts of your appliance off, hair dryer area then reapply appliance. Also okay to shower with waiver only. Or any combination. Good question.
Colostomy. Some patients cover their colostomy with a watertight dressing such as a band-aid or cellophane and tape.