No, it is just a small additional detail for this cosplay. It is not an essential part, either. There are two different Yumeko cosplays. The first one is Yumeko when she is being a regular school girl. The second one is Yumeko when she is gambling. When Yumeko is a normal school girl, she has regular brown eyes. When Yumeko is gambling, her eyes turn red. So that is when you can wear the red-colored contact lenses.  I do think gambling Yumeko is the best-looking cosplay out of the two. However, you will need safe and high-quality contact lenses to make it safe.