White shirts are for timekeepers and other volunteers.  Red shirts are generaly for senior officials.  Check with your team organizers if you are unsure which color you need.    
 
6.3-oz, 100% performance polyester interlock with wicking technology
No Bleed Fabric (NBF) created with a unique cationic dye process for easy printing
Rib knit collar
Moisture wicking
Breathable
Tagless
Classic fit