Do you see what he means (in detail, not just generally) or can he give you specific examples? Including what he would have liked you to do differently in a specific case?
From what you've said, I can see how you might be perceived as "unwelcoming" (as in, not proactively welcoming) but if you're saying good morning and responding politely if they ask you something or need your help, I don't see how you're "rude". "Rude" is a word I would very much avoid as part of feedback to an employee, especially in a formal performance evaluation, as (1) it's a loaded term and (2) it's very subjective/culturally-specific.
If it's new colleagues specifically, perhaps all that's needed is to welcome them to the organisation when you first meet them, say you look forward to working with them, chat for a few minutes about your role in the organisation, and invite them to come and ask for your help if needed in the areas you're familiar with. Maybe check in in passing with a quick "how's it going?"
I wouldn't go too far with this though unless you've been specifically assigned as a liaison or hands-on trainer or similar for the new employee; it's the new employee's manager's job to provide onboarding and training support and answer any questions/deal with issues. The manager needs to make sure s/he has control over what is being communicated, and you have your own work to get done. And if the manager is expecting/requiring you to chat about personal things or non-work-related areas (beyond the usual casual "how was your weekend?" "fine, thanks") level, then HIBU. Some people like to do that, but it shouldn't be an expectation or a requirement.
Do you have other longer-term colleagues that you're friendly with? Perhaps you could casually ask one or two of them how they think you come across to newcomers, or if they find you brusque in some interactions at work - you don't have to specifically say that your boss raised a criticism.