Colouring my Chocobo...what am I doing wrong?

[Ashkendor]

So when it says 6 hours I shouldn't feed it again but it only randomly actually changes? Because I've been through this twice now with the waiting

Any time you feed the chocobo it starts a 6 hour timer till when any change would take place, regardless of whether or not you actually fed it enough to make it change. You need to keep feeding it the fruits to make it your desired color until you see "(Chocobo) is growing new feathers!" in the chat box after a feed. That's the indicator that you've fed it enough to kick it into a new color range. You can either try to keep feeding it for more intense color results (a single message will likely only result in a very slight change in color, like Desert Yellow > Millioncorn Yellow, etc) or you can wait the timer out and see where you stand before proceeding.