I think you'd be safe when modeling an M36 to exclude pre- and early war ammunition.
I've uploaded a color chart to my gallery that ought to help. In short:
M71 HE - OD w/ yellow markings
M77 AP-T, T33 AP-T, T30E15 HVAP-T, T30E16 HVAP-T - Black w/ white markings
M82 APC-T - OD w/ yellow markings
(The M82 APC-T round would is OD, not black, because the bursting charge made it an explosive projectile.)
None of the AP type rounds appear to have used a steel case, but it was common with the M71 HE.
HTH,
KL