Like I said before, the Crucible in Destiny 2 will show what subclasses people are using by displaying their subclasses' symbol overhead. If they were to add the option to change the color for subclasses, they could also have it so that the symbols of the subclasses would would match the color you got equipped. That way in the Crucible, everyone can see what subclass you are as well as what color you have set. : )

I get that But, for example, if a blue Hunter turns the corner and a player thinks it's an Arcstrider, it will surprise them if that Hunter shoots Golden Gun bullets. Or if a purple Titan turns the corner and smashes them. That wouldn't go down so well

I don't think the purple Titan was the best expample since Sentinals have close range capabilities like Strikers. I would suggest a purple Dawnblade instead, but I digress. I would argue that when it comes supers, regardless of color, you're likely gonna die if you bump into one around the corner. Your best chances are keeping your distance. : )

True, at the end of the day the colour makes no difference because you will always die, but the experience is what matters, keeping it fair, you know, not confusing.