Replace internal colors with avh4/elm-color

[MartinSStewart]

In avh4/elm-color's own words: The this package defines a standard Color type (taking the place of elm-lang/core#Color in Elm 0.18) with the hope that all Elm packages that produce colors and all Elm packages that consume colors will use this type to allow all such packages to easily interoperate for the ultimate benefit of all Elm developers.

For me this is already an issue as I have both this package and elm-ui in my project and it's annoying needing to convert colors between the two.

[mayoff]

For the lazy:

https://package.elm-lang.org/packages/avh4/elm-color/latest

https://github.com/avh4/elm-color/tree/1.0.0