Vial of Endless Draconic Scales
Source: Vendor. Costs 40 It looks like a manuscript but when used it adds a new spell to your spellbook, just expand the dragonriding option and it will be there.
The 
Vial of Endless Draconic Scales is a toy purchased with Riders of Azeroth Badges that grants your mounted dragon a randomized appearance for one hour.
Guide awaiting review
Locate the Vendor
Find the Riders of Azeroth vendor who sells the Vial of Endless Draconic Scales during the Riders of Azeroth World Event.
Purchase the Toy
Buy the Vial of Endless Draconic Scales for 40 Riders of Azeroth Badges, then add it to your Toybox and activate it to receive a one-hour buff that randomizes your dragon's appearance whenever you mount — note that this buff does not persist through death and must be manually refreshed each hour.
Tips
This toy is currently character-specific and does not unlock account-wide, so you may need to purchase it separately on each character you want to use it on.
