Many churches hang a purple cloth on the cross to represent Jesus’ royalty as King of kings, and to remember the time when the soldiers mocked him and and put a purple robe on him during his trial. The purple cloth is traded with a black cloth which is hung on the cross for the final week, representing Jesus’ death. Then on Easter morning, they change it to white, to represent the cloth that wrapped his body, but was left in the grave when he rose from the dead Easter morning.
A crown of thorns can be added to the cross and cloth, or combined with flowers and a cloth for a separate decoration for the church.
Link to website that sells 2 sizes of crown of thorns: