Firefox breaks on the Legend tests

Discovered by @kwcantrell in #400. Impacts the `addCategoricalKey` test. Kalen's comment copied below:

```js
// Note: firefox breaks on this test. chroma.js does not
// recognize "rgb(255, 0, 0" on firefox. This is not an
// issue for empress as we would not pass "rgb(r, g, b)" to
// chroma.js
```

Start with the addCategoricalKey test mentioned in the issue and reproduce its behavior in Firefox, comparing it with the other browser results. Done means the Legend test passes in Firefox without breaking the existing test behavior.

Written by the indexing model from the issue text.