So that symbol refers to the “element” of the weapon, of which there are various shields that have the same color. Matching that element does bonus damage and makes the shield explode. Matching is crucial when the modifier “Match Game” is active due to the shield taking little damage from anything but the matching element. This idea also works with your subclass choice. Guns such as Dead Messenger, Borealis and Hardlight are capable of switching to accommodate any elements, while Arbalest has a unique function of being able to efficiently break any elemental shield in the game. Sometimes, matching your guns with your subclass can be beneficial due to synergy that you can create, either through perks such as Incandescent, Voltshot, Repulsor Brace, or Headstone. There’s also a mod called “Font of Might” where if you pick up an elemental well that matches your chosen subclass, it gives you 10 seconds of 25% increased damage with any weapon you have on that matches your subclass element. In regards to what race has what shields, one of the guys on here answered it most of the way, only neglecting the shields that Taken have. More often than not, when you go into a higher level activity, it will tell you in the modifier readout what shields you will you will find, which is helpful of you don’t have a clue who has what shields. I think that about covers it.