There is no standard for wire colors on 3-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 3-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 and second 3-way switch together. 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.