There is a feature in Slack called emoji reactions. It allows users to respond to any message on Slack with an emoji reaction.
(source: slack.help)
Also, you can choose a skin tone for your emojis:
(source: slack.help)
If you do, this
will turn into that
If multiple people with different skin tones selected react to the same message it will look as follows:
If you want to support someone else's reaction you can just click on it.
Now to the question itself. Let's say there is a message with a reaction from someone of different skin color. What should I do if I want to add the same reaction myself? Both options seem bad to me. If I react with a reaction of my skin tone, it will be shown as a separate reaction. It will look like I want to put emphasis on my skin color and separate myself from whoever reacted the first. If I react with a reaction of the skin tone of whoever reacted the first, it can be seen as mockery or cultural appropriation. I'm really at a loss here.