You defined the ‘kleurtemperatuur’ item as a standard dimmer, and that will not understand anything other than ON/OFF or a value between 0 (=OFF) and 100 (=ON) AFAIK. It is basically a percentage.
For most light bulbs that support color temperature like IKEA TRÄDFI or Philips/Signify Hue, there is support in the binding for color temperature. For instance, the Hue binding translate the value for color temperature to a value between 0=‘cold’ up to 100=‘warm’. This is not going to help your configuration as it communicates via a generic MQTT topic.
You could define the item as a Number and use Setpoint in the sitemap; may not be very user friendly. Also a ‘proxy item’ could work, combined with a rule that converts values, or even a simple transformation of values.
It may be advisable to start creating predefined values (or ‘scenes’ as they are often called) to simplify the UI.