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Given an MOS scale, the Chroma for that scale is the interval that is the difference between the large step and the small step in the scale.
For example, in the diatonic scale the large step is the major second (C-D) and the small step is the minor second (C-Db). The difference between those two intervals is the augmented unison (C-C#).
The term was suggested as a way to generalize the concept of a sharp or flat (# or b).
Moving one note in an MOS scale by a chroma leads to a MODMOS (if the result is not just a transposition of the original MOS scale.)
See also 25/24 the 5-limit chromatic semitone.