I hate to agree with you. This honesty/positivity dilemma is something I'm struggling with.
1. Focus on something going well. 2. By the way, could you fix this little thing? 3. Return to positive stuff.
This formula can be applied to delivering honesty amid positivity. Ideally whoever you're communicating is mature enough to just take honest feedback straight, but if they aren't then this can be a safe(r) way to deliver it.