The colors of the color-coding system represent whether the week is considered a peak week at a particular resort. Red weeks represent "hot weeks" and generally carry a higher exchange value than other weeks. Yellow and white weeks are neutral weeks, meaning they are not considered prime/peak weeks or off-season weeks and carry moderate exchange value. Green and blue weeks are considered off-season weeks, with low exchange value. Because owners of red weeks can request an exchange at any time of the year, they have more levity when exchanging weeks compared to owners of yellow, white, green or blue weeks.
For more info, see 5 Things Timeshare Exchange Companies Don't Tell You