I would love new colours to be added to the game providing they don't hamper the ability to cast and tell apart players at a glance, as you mention.
While the new colours are very nice to look at, it cannot be said that they're different enough from the existing ones.
For reference, this is the current list of colours:
This is the proposed one:
While the original list of colours isn't perfect, the new one is much worse. If all of the most similar colours (neptune turquoise vs cyanide cyan, peachy orange vs barrel bronze vs cider yellow, pale sky vs positively pewter) were removed then I think this idea would have much less opposition. As it stands right now, though, I am against this change.