Thanks Baloo - I did find that I could go into Developer > Forms > Data Items then choose System and then Reactions from the second field. If you click the edit button there, you can see the colors assigned as well. For example, "Love" reaction looks like this:

a:2:{s:4:"icon";s:26:"heart sys-colored col-red1";s:6:"weight";s:1:"1";}

I do not understand that enough to make changes outside of red1, 2, 3 etc. Seems anything else I do breaks the reaction emoji. I am hoping someone will tell me how to add the ones displayed in https://github.com/unaio/una/issues/1691