The task section presents all your reminders for the day in Priority order. By setting things to be A,B,C priority, you can move reminders up and down the list.
Once an item is marked as Done, the priority will be changed to X to show its completed status.
If you want to change the color associated with each of these letters, go to Options, then Tasks. You can pick a priority letter from the list, then match up the color you want to associate with that priority.
The same Options screen would also let you add your own, but keep in mind this field is for sorting only, so priorities should only be 1 or 2 letters long. If you want to indicate what type of task these are, use the Groups field instead of priority. You can also right-click on the column headings to make Group visible as one of the columns in addition to the ones you already have.
Do you sync your Tasks with an iPhone through ChaosHost? If so, keep in mind that the mobile devices have a much more limited sense of priorities. For devices that sync through Exchange ActiveSync, you only have High, Normal, and Low as priorities. For synchronization purposes, High = A, Normal = B, Low = C and Completed Tasks = X. If you use any other priorities, they will be synced as Normal priority as well.
This help article applies to Intellect 4 or newer, Time & Chaos 8 or newer, and Chaos Free