Allow modification of built-in skincolors

Modification of built-in skincolors was considered problematic by a former developer, so it was removed during development of !960 (merged). Given that this person is no longer a member of STJr, it was suggested that I reintroduce this feature.

Additionally, minor changes were made:

• The wording of an error was changed to make it more consistent with other errors.
• Previously, an error would be shown only if the length of a skincolor ramp was less than expected. This has been changed so that the error displays when the length is greater than or less than expected.