Quoted from J85M:
Mark III was the first none prototype armour, so it’s significant in that regard as his first true “Ironman” armour, which makes Lymans new bonus pretty great and shows he just gets how to integrate a theme.
But still a blue insert is just wacky visually, I always liked the white as I looked at the mark inserts as those suits arc reactors, building up to the mark 6 reactor, which is significant again as it’s the first armour that runs off his new arc reactor, that he designs in order to cure himself from palladium poisoning caused by his version 2 arc used to power armours 2 to 5.
Either the rules/inserts are a major stroke of luck or it’s some amazing little bits of attention to detail from Lyman integrating the code around the theme.
That feels like a bit of a long bow you'd concluding regarding theme integration.
The blue is definitely wacky and so far out of theme its jarring.
Why blue? Why not red or yellow seeing that's the suits main color themes.
So I re-read the change note line to see if there is any other clue.
"Added a half-way bonus for the MARK SUITS. The bonus is lit after the third MARK SUIT and awarded at the SPINNER."
I do agree however on re-reading this that it might be a way to make a "blatantly obvious visual marker" to the player about the new half way bonus.
Aesthetically it looks nuts and seems like a really silly choice to bake it into the hardware.
So perhaps as they don't have color changing led's this was what they thought was their only on playfield way to highlight "bonus is lit" or that there is something to get to when its not lit.
A unique blinking pattern would have been easier, works on all machine revisions and would still draw the players attention when available.