The difference between compliment and inappropriate behaviour is in the way you do it, more than what you do.
You can easily compliment people, by
- Making clear through your behaviour that this is what you intend (and nothing more), and
- Anticipating that they may be worried anyway, and so making very clear by your manner, that is all you intend.
What follows is an actual example I had myself, a few days ago. I happened to see a young lady with really nicely done hair, at a train station. I wanted to compliment her (a stranger and young female POC) without triggering concern. My motive was that I felt she might like this, so long as it didn't worry her.
I just said as we waited near each other, "I just wanted to say, your hair looks amazing", I smiled briefly, and then almost immediately, I started to walk about 20 yards up the platform and began reading things on my phone, looking towards the train tracks not at her. I got into a different carriage, and didn't see or speak to her again, or even look for her.
She got the compliment, and also got the message that I truly had no ulterior motive. Because:
- I didn't try to convert the compliment into a conversation, or use it as a "hook" to engage her any further.
- I didn't try to stay in her personal space, or near her
- I didn't try to watch her for any reaction
- I immediately turned my interest back to normal disinterested passer-by things, like my own phone, and my expected train, nothing about her (as if we had never spoken)
- I walked away "normally", like anyone walking down a train platform. Not furtively, or guiltily. Just like normal. But not too slowly, and not looking back or anything.
- I didn't try to pick it up later.
- I didn't act like I had done anything wrong, or weird, or that I needed to explain anything to her, or carry on talking to apologise, or explain it was safe, or anything. I just shut up and left well alone/walked away, saying nothing more.
I think this last point is really important. It's so tempting to explain, but its actually an extended engagement that is most likely to cause worry (whatever is said or whatever the intention!) than anything else. Trust that skipping it, and skipping any other hint of further interaction, will work better than misguidedly trying to "make sure it's okay". If due to aspies or any other reason, this doesn't make sense, trust it anyway. It's very important.
In an office setting you can do the same, with modifications.
- say "that looks nice", and nothing else. Say it as a compliment, but keep it bare, don't embellish, don't add more, and make clear what exactly you are complimenting.
- Then immediately, but without acting guilty or furtive, or rushing, go do something of your own stuff. Some work, or a file that's elsewhere, or a coffee or whatever.
- Don't explain, expand or apologise, or try to say anything more.
If the colleague wishes, they will say thank you. If they don't, they won't. If they do, you can say, "I didn't want to say more in case it wasn't wanted". Then again shut up, they will comment if they wish, if it was fine. But most people in my experience will be OK with a very brief compliment, as there isn't time to get anxious or worried before it's over - and it's clear the person really didn't mean or want anything more.
In a separate incident yesterday, I got to see the reverse side of this. I was walking down my local high street with an LGBT friend with a new and frankly awesome haircut. A passerby - a complete stranger - walking on the same side towards us, said as they passed, "Love your hair!", and said nothing else and carried on walking without stopping. My friend, who is quite stranger-phobic and anxious socially, and would normally worry, was left smiling by the compliment and the way it was done, and felt good.
In today's society, that's probably the safest way to do it, apart from simply not doing it.... which is always safe.