Nora's name and colour comes from a flower, the Nora Barlow Columbine flower. Neptune's colour comes from the planet Neptune, which is blue with other hues.

No one said names couldn't have more than one meaning behind it.

Also the qualification for the naming rule is just it has to be any kind of colour, whether or not that colour is imaginable is another thing entirely, as long as it qualifies as a colour. That's why Ooblec counts.