Some colors supported by three.js are not allowed by traitlets

[moorepants]

For example if I set a material color to grey in pythreejs I get an error and have to change it to gray, but you can see that three.js supports both grey and gray: https://github.com/mrdoob/three.js/blob/master/src/math/Color.js#L7.

[vidartf]

We are reusing the color trait from ipywidgets core. We could subclass it in pythreejs to add a few more colorwords (or this could be done in ipywidgets if these are commonly accepted CSS colorwords), if someone submit a PR implementing it :)

[moorepants]

Ok, I'm trying to swap out pythreejs in PyDy for our custom three.js wrapper and this issue came up. I'll look into it when I'm next working on it.