It's either a pink sticker or a green sticker. It's random but can't say if there's any bias or not.
Maybe they are more likely to test people from certain countries. I am from India, and I think there were cases of people trying to present fake vaccine certificates/test reports at some point in the past or maybe in the past people arriving from certain countries were tested positive more than other countries, so they test people from those countries more often.
I vaguely remember that the immigration officer had only pink stickers on his table when it was my turn. Could be he only went through a certain color at a time, could be he had only pink ones and a different officer had only green.
Who knows. But for all intents and purposes, it's random.
If random testing is still going on before you arrive, I would suggest keep wearing masks and follow social distancing 2-3 weeks before your flight, if possible, to stay safe. I had a COVID scare in my family a month before my flight. So, I had regular tests as well to just keep track. This was my 3rd attempt because of the lockdowns. Didn't want to risk anything