Currently, when reviewers select the "Ship It!" option, the notification sent

to Slack has a bar that is colored grey. I found that a value of 'success'

is passed to color. Upon investigation it seems that Slack and Mattermost

doesn't understand what color corresponds to 'success'. However, it knows

'good' corresponds to green.
Evidence that Mattermost uses ‘good’ and not ‘success’:

https://mattermost.atlassian.net/browse/MM-3181

https://mattermost.com/blog/getting-started-with-mattermost-integrations/
Evidence that Slack uses ‘good’:

https://api.slack.com/reference/messaging/attachments#fields

I ran the unit tests on Slack and Mattermost. Two unrelated tests are failing

because they cannot find logo.png at /var/folders/.... See #integrations
Confirmed that "Ship It!" message has a green bar instead of a grey one in 

Slack and Mattermost for Review Board version 3.0 and 4.0 after the change