A cosplay wig is not just hair color. It is silhouette. The fastest way to improve one is to study the character's outline before buying anything.
Choose the base
Pick a wig with enough density and length for the final shape. If the character has spikes, volume, or extreme bangs, buy more hair than a normal style needs.
Add structure
Use teasing, crimping, steam, glue, wefts, wire, or foam depending on the style. A steamer is one of the most useful cosplay wig tools because it resets synthetic fibers.
Cut last
Build volume first, then trim. If you cut before shaping, you may lose the hair needed to cover structure.
Bottom line
To make a cosplay wig, build the character's silhouette first. Color matters, but shape sells the costume.