Thank you Shawn.. I didn't know what was I thinking... it was that simple. Thanks.
No option to mark two correct answers for the question!!?
Thank you Joe. I think the medthod you provided will come in handy.
Here's how to do it without having to figure out what was done (aimed at Tableau Padawans who aren't as familiar with the tool). First, make YEAR a Dimension (it's currently a Measure).
Drop the YEAR into the Color pill box. Right-click in the Year box that appears and select 'Edit Colors...'. Click on a year in the 'Select Data Item:' pane and then click on a color in the right pane. Repeat. Click the OK button when you're done.
Right-click in the Dimensions pane and select 'Create Calculated Field...'. Name the field "Year Color" (no quotes). Use this as the formula:
IF [Year]<=2012 THEN
"2006 - 2012"
"2013 - 2020"
Click the OK button. Now, just like above, drop the 'Year Color' into the Color pill box; right-click in the newly appearing 'Year Color' box (or click on the drop down arrow in the upper left corner of that box) and select 'Edit Colors...'. Select your year category in the left pane and then click on the color you want it to be in the right pane. Click OK when you're done.
I hope this helps to explain how the given workbooks were made.