I seem to recall that in the late 80s/early 90s, around the time the BEP starting planning the current redesign, there indeed were proposals to go to multicolored money, with the argument that it helped make bills less easy to counterfeit. (This was before color copiers were so prevalent.) My impression was that the proposal was dropped mianly because the BEP didn’t want to deal with the masses of 'merkins who hate anything other than a greenback.
You are doubtless referring to Cecil Adams’ column "Were magnetic strips put in U.S. currency so the feds could track your money? (24-Jun-1994)
Coincidentally, there is a discussion going on in our “General Questions” section on the same subject:
Some people are saying that multi-coloured bills might help deter counterfeits, others say it would be a needless expense. With the advent of colour copiers and printers, however, it would seem that multi-coloured bills are not the most effective deterrent.
There is not total agreement, however, with your suggestion that “intertia” is the main cause for leaving intact the colour of USA currency.