There is no standard for wire colors on 3 and 4-way switch travelers. The colors will vary depending on whether NM cable or conduit was used.
With NM cable, the wire colors for travelers will be black and red using 3-wire cable.
With conduit, the wire colors for travelers could be any color. In the 2nd diagram below, blue wires are used for travelers.
Typical 4-Way Switch Wiring Diagram, NM Cable
Typical 4-Way Switch Wiring, NM Cable
In the 1st diagram below, a 2-wire NM cable supplies power from the panel to the first switch box. The black (line) wire connects to the common terminal of the first 3-way switch.
A 3-wire NM connects the traveler terminals of the first 3-way switch and the first 4-way switch. A 3-wire NM connects the traveler terminals of the first 4-way switch and the second 4-way switch, and so on. A 3-wire NM connects the traveler terminals of the last 4-way switch and the second 3-way switch. Traveler wires are interchangeable on each switch.
The common terminal of the second 3-way switch connects to the light fixture(s).
The white neutral wires are connected together in each switch box.
Diagrams shown on this page are simplified for clarity. Electrical outlet boxes can have numerous NM cables going in and out. See Actual Switch Box Wiring Diagram.