as Rex says:
“The colour coded knobs are an option for newly ordered M-class m-odules and Shop series panels and are provided to our STS clients at no additional charge.
Retrofits to systems already in the field will be handled by payment &
exchange arrangements between STS clients and myself addressed on per case
look at the photos section of the SMOG yahoo group (in the m-class series folder) for my CV processor and Wave Processor m-odules that follow this knob colour scheme.
Black knobs are for offsets and audio (AC) attenuators, and Blue Knobs are for cv (DC) attenuators.
(the Red knobs are just added to my m-odule for fun, they are not included in the new knob colour scheme.)
Knob color schemes aren’t a big deal for most modular synth owners – but if (or when) your modular synth starts to get large, color coding can be handy.