Google Drawings

Google Drawings is an art app (accessible through Google Drive).

Lines
Google Drawings has several types of lines, such as line, arrow, polyline, and scribble.

Tips
For coloring in refs, I suggest using either line or polyline.

Shapes
Google Drawings features many shapes, including circle, trapezoid, octagon, triangle, and many types of word bubbles, banners, and arrows.

Tips
Working with shapes can be difficult, as it is sometimes SO FREAKING DIFFICULT to select EXACTLY THE RIGHT SHAPES that are EXACTLY THE RIGHT SIZE. However, the results can be good, particularly if you are making an inanimate object.

Colors
There are many different color options, and you can choose linear or radial gradient, and choose custom colors and gradients.