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.
English
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If it were to be done in a limited a sense, I would propose it being similar to ornaments in that each subclass has 2 additional but distinctive color choices depending on their element. Arc: arc blue, crimson red, or electric yellow Solar: solar orange, hellfire green, or charred black Void: void purple, silver white, or starlight gold pardon the color names. ; )