The first LEGO axles were milky white and were probably made in
a different plastic than they are now. Later on, LEGO started
making all-black axles when they started doing more serious
technic sets. However, the tendancy is now to make sure all
difficult technic pieces are "color-coded" so that children can
pick them out more easily. As such, nowadays, the following axle
lengths and colors are standard: - 2: red, notched - 3, 5, 7, 9:
(new) grey - no 11 yet as far as I know but I suppose it's just
a matter of time - 4, 6, 8, 10, 12: black - 3 with stud: dark
tan - 4 with stop, 5.5 with stop, 8 with stop: (new) dark grey
Similarly, 2L friction pins are black, non-friction grey, 3L
friction pins are blue and non-friction tan, and so on. All that
to make sure they're easy to pick. Even gears are being changed
now, for example the classic 24T is now dark grey. That said,
there have been other colors used for axles, when the color
itself was important. I've seen tan, red, green, brown, but
possibly others exists.