There's a whole lot of regulation around this. The UCI would have to agree, as would the race commissaires. You may recall what happened when LA's team decided to change kit for one day - the unedifying sight of riders changing at roadside, replacing numbers, extension of neutral zone, shambles. There's also the matter of political slogans and emblems being barred from kit. Whether "Mandela Day" is political or commemorates a towering figure, it's may still be seen as political in a way as there are ANC and communist party connections. All very messy and complicated. And as has been noted, opens a can of worms. Other teams may not object, but use it as precedent for their own kit changes, and the whole thing would be out of control. An impossible situation. A subtle change kit for an entire race is a different matter - old hands will recall ONCE doing this every TdF, changing their base colour from yellow to pink to avoid clashes with leaders jersey, but that worked for everyone. The Sky blue/green swop was again for the DURATION of races, not an odd day, and essentially apart from green detailing instead of blue the design did not alter.
So whatever anyone might think is OK may well not be. It's just possible that a modified design for the entire TdF might get approval.
Everything is a lot more complicated than it appears from a simple forum opinion!