Are you viewing this sitemap in an app, or in BasicUI?
Sitemaps don’t have anything to do with the usual beginners starting point, the main admin GUI,“MainUI”.
Aha, now this gets used in MainUI. So, either/or, which are you working with?
Yes, you will need a Thing configuration.
I suspect some openHAB basics have gone over your head until now.
Items are the heart of openHAB, idealized models of simple devices.
In this context, all Color type Items are alike - RGB,HSV,LED,zwave,MQTT all make no difference.
Because they’re all alike, we can use consistent UIs with these Items. And there is a choice of UIs.
To use a UI widget, just click it, it sends an idealized standard command to the idealized standard Item.
Then Things and channels come into play - Things are a rough model of someexternal device or service. Channels link the Thing to Items, perhaps many Items with different properties. Things and channels are configured to matc the real-world variables and weirdness to the standard model Item. So here is where RGB mode, or HSV mode etc. become important.
So Thing and channel settings decide how and where the standard command gets converted to bits and bytes and sent over wire or radio or whatever the real technology is.
From the above you can see that MQTT and Colorpicker widget have no direct relationship at all.
Make a Colorpicker work with a Color Item? Sure, works out of the box. How to set up is dependent on the UI that you choose.
Important limitation here though - colorpickers, like a slider, require a sensible starting point. Might not work, might not even be visible if your linked Color Item has not got an initial value, from device, or by other intervention.
Something you might gather from the above … “but I haven’t got a Color type Item!”. Yep, that’s your problem. You cannot use the standard colorpicker with a collection of Number Items, with different colour components
Your MQTT settings don’t look silly, but not enough to say if they suit the real device. They certainly don’t suit a Color Item.
What you need to do is combine the incoming MQTT data elements into a single color channel.
Might help-
otherwise you might need a transformation to assemble x y b values into one.
Once your Item state is updating correctly, then worry about colorpickers.
To get colorpicker generated commands out to MQTT, this looks useful