So long as the property is returned to its original condition, less fair wear and tear, at the end of the tenancy then it's nothing to worry about.
You'd never be able to get an eviction based on their taste in decorating alone.
However I'd be reluctant to renew a tenancy where a tenant so blatantly ignored the terms of the agreement.
On a practical level however, I'd be slightly concerned about the ability to paint over garish wall colours with neutral colour paint - or at least a tenant's willingness to do so. I hope the LL is holding enough of a deposit!!