By default, Basic HTML strips <span style> attributes, which CKEditor uses to apply color.
On /admin/config/content/formats/manage/basic_html, if you add <span style> to the Allowed HTML Tags, the buttons will be displayed.
This doesn't yet have official support in the Drupal Extension. The linked thread has some examples that use traits.
However, I found this gist by johnennewdeeson to be a faster, easier solution for me, just cut and paste into FeatureContext.php:
* @Then I fill in wysiwyg on field :locator with :value
public function ...