Kiva: document colors and how they are used

[corranwebster]

Add user documentation describing:

• how colors are represented (eg. 0.0-1.0 rgb(a) tuples)
• fill colors vs. stroke colors

[rahulporuri]

For now this information will be added to the the Kiva Quickref docs. After this sprint, we will reevaluate the kiva documentation to see if the information can be better organized.