Nixie tubes currently don't respect the new lamp color setting in 2.0. I managed to get it to work by changing one line in setStates, control.lua:138, from:
if not color then color = {r=1.0, g=0.6, b=0.2, a=1.0} end
to
if not color then color = nixie.color end
Though the color only changes after the value updates. But there seems to be a new event in 2.0 for detecting color changes.

Wow this is very helpful! The small amount of time it's incorrect while changing is nothing compared to having full RGB control the tubes. Should be added to the mod right away.

it works fine: you have to select "use color"

I have just tested pressing "use colour" in the current version with your original unmodified code and it doesn't work. The colour still remains orange as long as I don't add teemu's new code. Honestly I would be surprised if it DID work because it's literally a different UI element, why would the same code work for the new RGB selector? Let me know if I'm wrong, but I think you're referring to the old "use colour" where you needed to send a number for Red, Green, and Blue signals into the tubes from a CC or something. Me and teemu are talking about the new feature that was added where you can select any colour without the need for a signal to set it. This new RGB selector is to the left of the "Circuit connection" panel. I am completely open to the possibility that I am wrong here and I didn't do it right, but from my tests it simply doesn't work without the new code.