It’s funny how mixing pure blue (#0000ff) with pure white (#ffffff) makes it look a little purple, and how you also have the exact same problem when you invert the colors - mixing pure yellow (#ffff00) with pure black (#000000) makes it look sort of green (the complementary color to purple). In both cases you have to adjust the green channel a little “faster” and the red a little “slower” to get something that looks more neutral, ie shift the light blue towards cyan and the dark yellow towards red.