If the voltage drop across the white LED is something other than 3.5V, then the voltage drop across the current-limiting resistor will change, and therefore so will the current through the series pair of LEDs. If the LED has a higher voltage drop, then the current through the resistor will be reduced and the blue LED in series with the white one will become dimmer. If the LED has a lower voltage drop, the blue one in series will become brighter. No matter what, the current through the white LED will be the same as the current through the blue one it's in series with, so with that schematic, you won't be able to run the white one at full brightness without blowing out a blue one.