Re: Plastic bumper colour doesnt match metal bodywork colour.
I have over painted automotive seam sealers when having replaced panels and I've never noticed any cracking of the paint. Though I wouldn't like to rate your chances of being able to sand seam sealer flat to give you a flat join less look.
Though obviously what you are attempting to do is going to clearly visible. It might be better to try and use a plastic filler used for bumper repairs that sets rigid ?
Neither would I rate your chances using a touch up kit ? I think you would have to spray all the panels.