Fix bug with boolean values for color

[thuiop]

Fixes the bug from #3106 by using a Nominal scale for boolean instead of Continuous.

[mwaskom]

I think this should be fixed in Continuous by making sure theres a cast to float before the subtraction. Then I also think there could be a Boolean or Binary scale type. I'm not sure about defaulting to Nominal.

[thuiop]

I do agree that there could be a Boolean scale ; however, I do feel that boolean data is more at home with Nominal rather than Continuous as there is a discrete number of values.

[mwaskom]

In any case, this is not the right place to add the default scale because color properties are not the only ones that run into the problem.