You would do your normal pass, of just the model lit however you want it to be minus the spotlight.
Then do a secondary pass where all the light is turned off but your spotlight.
Once the secondary pass is done take both of the results and combine them in a postproccess where now you have control of the color values.
https://playground.babylonjs.com/#QFGSM3#1
There are a few steps missing but this should get you there at least with the postproccess way.
I can think of a half-dozen different ways to solve this so if you run into a road block let me know.