 Mistakes were covered with various forms of correction fluid, including the type used on typewriters.Watterson was careful in his use of color, often spending a great deal of time in choosing the right colors to employ for the weekly Sunday strip; his technique was to cut the color tabs the syndicate sent him into individual squares, lay out the colors, and then paint a watercolor approximation of the strip on tracing paper over the Bristol board and then mark the strip accordingly before sending it on.When "Calvin and Hobbes" began there were 64 colors available for the Sunday strips.