In the binding you have to define the acceptable item type for a given channel within a thing. This is e.g. "Dimmer" for the "brightness" channel or "Switch" for the "switch" channel. PaperUI uses this to allow only item/channel-links that are valid. Therefor for linking a "Color" item to the channel of a thing you need a channel that has set the item-type to "Color".
The Dimmer thing is a thing that can contain an arbitrary number of DMX channels, controllable via the "brightness", "switch" or "color" channel. This color channel was not implemented, but it is now.
Didn't see that. How many DMX channels do you use in total? And what hardware do you run OH on? This is most likely because of a timing issue. It could happen because all channels are checked sequentially for changes. However, it should not, because the time used for the value calculation is set before each channel calculation is called (at least in the new binding, not sure about the OH1-version). My OH2 instance runs on an APU2, so there is quite a lot of processor power, compared e.g. to a RPi.
@All: If you want to test, the latest&greatest alpha is always available here.