In most cases stop signs on private company roadways or within a company building are not required, and posting a stop sign would simply be going above and beyond OSHA requirements. However, if a stop sign is used, OSHA requires that it be red, in accordance with 1910.144(a)(1)(iii) and uniform with other company stop signs.
Note that OSHA calls for warning signs and traffic control signs at 1910.268(d)(1) for the telecommunications industry.
Also, 1917.44(e) requires stop signs at marine terminals.
Finally, 1926.200(g) calls for traffic control signs and 1926.201(a) goes over signaling by flaggers.