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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Like some have already said, subclasses need to be immediately identifiable for gameplay reasons. But i'd like to see your idea in a limited sense. Maybe chroma within the subclasses colour spectrum. Arc=different shades of blue Solar= different shades of red. Void=different shades of purple.