White LEDs consist of a blue LED with a layer of phosphor on top.
When hit with blue light the phosphor emits red and green. This, combined with some of the blue light from the LED that passes through the phosphor is where you get the red, green and blue light that makes up white light.
If your LED turned angry blue after a failed de-dome it means you took too much off. Basically you accidentally removed part or all of the yellow phosphor layer revealing the underlying blue LED.