How did the serving in WWI differ for African Americans and Mexicans?

Well, if you mean in everyday experiences, a lot.

To start, African Americans were actually willing to fight for the U.S. when the United States first entered the war. While they were accepted into the military, the U.S. actually ended up giving them to the French because they thought they weren’t good fighters. However, when these soldiers joined the French, they would actually develop the name “Harlem Hellfighters” and be some of the most effective units of the French army. When they went back to civilian territory of the French, there were stories of people running up and cheering them on at their arrival. One even attested that a white woman kissed him on the cheek. This was way different compared to the United States... instead of being cheered, some of them were actually lynched...

I hope this helps, have a great day!