Once you have unlocked all traits for your artifact (including Concordance) the colour variation for that spec can be bought from a vendor in the class hall for 1k order hall resources
Also note that you need to be in the spec you're trying to buy the mount for. I.e. if you're Assassination spec, with concordance on your Outlaw weapon, you still can't buy the Outlaw mount unless you swap specs to Outlaw.