The Sims 2 - obtaining the CEP file
The creation of these packages wouldn't be possible without the joint efforts of Quaxi, RGiles and Numenor at ModTheSims (warning: ModTheSims is not exclusively PG-13 - by clicking this link you acknowledge this).
CEP is a Color Enabler Package. By default, The Sims 2 does not work with all recolors. CEP fixes this. Note: you only have to download and install this once (not for every custom content file you download).
For Windows, you can opt for either the automatic or manual install option. Mac has manual install only. Both download options contain full instructions:
Go here for the official download location and installation instructions.