Chromatic Intervals are the notes that do NOT appear in the Major Scale.
Chromatic intervals come from diatonic notes in the major scale that have been raised or lowered by half steps. Only notes that appear naturally in the major scale can be a diatonic interval. If it isn't a diatonic interval then it's a Chromatic interval.
There are three kinds of Chromatic intervals. Augmented intervals, Minor intervals, and Diminished intervals. Augmented intervals are Major and Perfect intervals that have been raised by one half-step. In other words, any diatonic interval raised by one half-step is an augmented interval.
Minor intervals are any major interval lowered by one half-step.
Diminished intervals are either a perfect interval lowered by one half-step or a major interval lowered by two half-steps. Notice the use of double flats. One flat lowers the pitch by one half-step. Double flats lower the pitch by two half-steps which is equal in sound of one whole step.
Augmented intervals are major or perfect intervals raised by one half-step.