Either way the "incomplete" forms of colour blindness are one of the most common forms of disability, so using color as the only distinguishing factor in consumer cable types is pretty exclusionary.
As long as you use fairly different shades of red and green, they can still match the cable to its matching color shaded port. Much UI accessibility is based on this fact.
Having to go even further than that and use shades of the colors I do have trouble with would be far more onerous.
Additionally in the physical world how are you going to determine the shade of something relative to something that is not present? i.e if you only have one cable in front of you. We don't have absolute perception, it tends to be affected by context, additionally the lighting now seriously affects your judgement... The reason this is so different to colour is because luminescence is a single receptor type (rods), with which you are trying to compare intensity of an area of one type of signal filled with other detailed information; whereas with colour you have three signal types (cones), in combination and at a lower detail than rods, so it's effectively separate information with at least 2^3 completely distinct permutations to differentiate in almost any lighting or context. This also explains why the absence of only a single cone receptor type has a significant impact on perception since it drops to 2^2 halfing the completely distinct permutations, of which two are merely black and white, so you actually drop from 6 to 2 non-monochromatic permutations - so you see it is far more challenging using either shade or partial cone types.
At some point there will always be someone who something doesn't work for. You suggest using symbols, but what about people who are completely blind?
So you use something like a combination of color and braille, and then the rare person who is monochromatic can use the same solution as someone who is completely blind.
