Simplify the following text for a Grade 10 native English-speaking person.
Original: Upon arrival at the camp , the prisoners were marked with colors , similar to the use of Nazi concentration camp badges : blue for Serbs , and red for communists ( non-Serbian resistance members ) , while Gypsies had no marks ( this practice was later abandoned . ) .
Simplified: Upon arrival at the camp , the prisoners were marked with colors , similar to the use of Nazi concentration camp badges : blue for Serbs , and red for communists ( non-Serbian resistance members ) , while Roma had no marks . ( This practice was later abandoned . )