The color stuff would be hard to pull off, Slack doesn't have styling for the center channel. Users in Mattermost can set different background styles for the center channel and if we generate some random colors, there are chances that they may clash with the center channel.
The extra vertical spacing allows to differentiate one message from another. We've got a bit of extra stuff going on, replies, multiple images, etc. If it was just messages having minimal space in between messages would be fine, but with replies and also the color restriction, the extra vertical space is there to help to differentiate between messages.
Also I think having avatars is a good idea, we have restrictions going on with the colors for different users, so avatars can help us with that restriction and allow people to differentiate between messages a bit more easily.