I am a muslim with a colostomy and find it difficult to be clean for prayers. what do you recommend?

Illness 'allowance': I am by no means an expert on this sensitive issue but most religions permit less strict adherence to ritual observance(s) in the event of illness - or in your case a chronic condition. So do the best you can and draw comfort from knowing you are doing your best. No-one can ever be expected to do more than their best..

Rubber gloves: The issue of clean and unclean in muslim culture is one of social convention as it is not stated in the koran. But the social conventions are strong - left hand for cleaning, right hand for greeting, etc. One way to help with this is to purchase rubber gloves and use them when dealing with the colostomy (a good idea anyway) and then you will be clean underneath.