Most DJ gear comes with black or grey knobs and fader caps. Several companies sell brightly colored knobs and fader caps that can replace the standard black or grey that your gear came with. In addition to looking cool, brighter caps can be easier to see in dark environments. Some caps have a hard plastic slippery surface whereas others have a rubberized surface that fingers don't slide off as much. Some DJs prefer plastic; some prefer rubberized surfaces. Be sure to check that the caps you order fit your controller before purchasing them.
Custom caps available:
Here is a video comparing different brands.