An alphabetic character, A-Z and a-z (i.e., both upper and lower case, 52 total) used to index an XTREE RGB color definition. The index is defined via Advanced Coldef Options RGBfg and RGBgb, and referenced by cformat codes B, b, C and c. A blank (" ") or "=" may be used to to specify the default color. Blank may be overridden by another explicit color specification affecting the tree component, such as a row or column color, while "=" will always use the XTREE default color for that component.