There are two different systems for describing Marriott weeks.
Interval International
Interval International ("II") classifies the various weeks of a resort as Red (highest season), Yellow and Green for exchange purposes. Thus, if you own a Red week, it should trade better than a Green week at the same resort.
However, there are many shades of each color that II doesn't publish. For example, a July 4 beach week and a mid-September beach week at the same resort might both be Red weeks, but the July 4 week will almost certainly trade much better than the September week.
Marriott
Marriott classifies weeks at its resorts in various ways. One typical method is to categorize weeks as Platinum (highest season), Gold, Silver and Bronze. Some resorts also have a super-high week or two designated as Platinum-Plus.
Another Marriott method used by a few resorts (e.g., Cypress Harbour) is to designate weeks as Special & Summer (highest seasons) and Sport (shoulder season).
A third Marriott method is a color system (e.g., for Desert Springs) of Red (highest season), White and Blue.
Marriott uses other classifying methods, depending on the resort.
Within the Marriott system of floating weeks, you can call ahead to reserve a week - but only within the season you own. Thus, if you own a Sport week at Cypress Harbour (weeks 18-23 and 34-50), you can only reserve your use within one of those specified weeks.