What are the different colors in anime production drawings for? In a lot of production drawings for anime (and I've also seen them in plots), particularly of characters as opposed to backgrounds, the detailed sketches/drawings have lines in different colors. Is there a purpose for this other than maybe to differentiate between lines? Are they used for color boundaries? Is there a production reason why the specific lines have to be different colors?









Edit:

Just to be a bit more clear, I was wondering specifically with the colors used in these particular style line drawings, where the lines seemed to be color coded. For example, given this animatic, the first sketch has color used in shading and hue, but the middle picture shows the color coded line art, where the tears are red and the eyes are blue.
The use of colored pencils and related mediums vary depending on the artist or production that does them.

The yellow is typically seen being used to highlight shadows or break up object in the background (like clouds), because shading with a regular pencil might drown out the details especially in a elaborate or darkly lit scene. Take this storyboard from Arrietty:



Compare it to this uncolored storyboard from Evangelion 2.0:



And the Steamboy:



Other times the yellow can be used to highlight an active foreground object like a person, take a look at this example from Mushishi:





The color blue is typically used as a secondary color, do add more detail and depth to the storyboards. Here is an example from 5 Centimeters Per Second:





Notice the blue is being used to highlight additional shadows for a better sense of the atmosphere of the shots. Here's an example of how it's used to separate objects in Gundam UC.



None of these guidelines are set an stone and can vary greatly depending the  budget and needs of the production and/or director(s). 

The opening storyboards for Ponyo are in watercolor: 



The main scenes too: