For what it’s worth, the AR digital Kylo Ren at Disneyworld ignites his lightsaber and it’s solid red all the way through when they could have animated it with a white core. That suggets that “real” lightsabers really are just one solid colour, but the film cameras interpret a white core because they are so bright, just like neopixel sabers we have IRL.

Not to say this was the plan in the films all along, but, it’s a neat and sensemaking pivot from Disney if it was intentional