I don't reckon it looks that bad. Put it this way, if you were only seeing the house for the first time (e.g. viewing with estate agent) I doubt you'd be thinking 'we like it but the roof tiles are a horrible grey colour.' So based on that, I'd just leave it as is, save the £££ on painting or replacing and chalk it up to experience i.e. not letting people pressure wash any part of your house 
