Conflict with $red, $green, and $blue variable names

I put together a project using the latest versions of Bourbon, Neat, and Bitters and I'm noticing that color variables with the names `$red`, `$green`, and `$blue` are overwritten by the Bourbon function, `is-light()`.

When I compile the following SASS (3.3.3) file, the color `$blue` is output as `202`, not `#477DCA` as defined in `bitters/_variables.scss`:
```sass
@import "bourbon/bourbon"; // Bourbon 4.0.0.rc1
@import "neat/neat";       // Neat 1.5.0
@import "bitters/bitters"; // Bitters 0.9.3

body {
  background: $blue;
}
```

It looks like the conflict happens when `@include button(simple, $base-accent-color);` is called in `bitters/_button.scss`. I'm using the default Bitters setup as an example but it looks like this could happen by defining a variable named `$red`, `$green`, or `$blue` outside the main SASS stylesheet.