It's a weird idea I know, but honestly why not? We already got shaders for your armor. Not to mention chroma and ornaments that can change the aesthetics for some armor sets and weapons even further. Heck, there are even some consumables that can change the appearance of your respawn for a little while.
So why not, eventually, have the option that would allow us to change the color of our subclasses' element/light? I think it would be awesome to see an Arcstrider wielding crimson red lightning in their hands, or the Dawnblade raining down hellish green fire from above, I'd even be curious to see what a Sentinel's shield would be like in golden yellow.
Like I said before, it's a weird idea but I thought it would be interesting to talk about such an idea.
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At first I was like, wtf? But then it sounded very cool. But I doubt it. I think for gameplay reasons it's important, as someone said below, for the abilities to be immediately identifiable when used. So the sounds they make are unique, and the way they look especially. If we start changing the colours, it may be hard for players to respond to their usages accurately or cause confusion. Really cool idea though.