That would be a real hassle trying to keep brown boots black with just shoe polish. That sounds like the Gov't/Army trying to save $$$$. And using up old stocks of left over WWII brown boots. Which I can see the "logic" in that. No matter what the effect was on the troops.
E.g. in the early '80s, we were still issued C-Rats left over from Vietnam. Until the stocks were all used up. And the MRE could be issued. Which again made sense, economically. Those C-Rats were good for about 25 year(?)IIRC. We even got old LRRP rats issued at times, again left over from Vietnam [Those were pretty good vs. many C-Rat meals !].
The Mess would even pack up brown bag lunches. Instead of issuing Cs or LRRPs for us to take to the weapons ranges. The formations had an M16, etc., in one hand and a brown bag lunch in the other.
In an uncharacteristic move. When the BDU came out. The stocks were not able to get enough for the entire Army to get issued. Plus the first batches were not of high quality. The black faded to purple and one arm or leg would shrink differently from the other. The troops would use black shoe polish to try to turn the purple back to black.
So the Army decided to make the "wise" decision to let troops wear a variety of tactical uniforms. In the beginning the old Fatigues, Camo Jungle or BDUs could be worn. Until the stocks were available for issue to all.
When I came back from the ROK in '86 at Ft. Benning, GA. We could wear the BDU, Camo Jungle or Green Jungle uniforms. And even the option of standard boots or Jungle boots.
"Old Timers"(?) like me(?) usually wore the Jungle fatigues and boots after having deployed to Panama 3 times. When I was with the 101, '80-'83.
Like I said a very "unique" move for the Army to allow such non-uniformity … in "uniforms" …