Ok, I'm really not familiar with enlarger's cold lights - that was my guess.
I do know that standard fluorescent lights 'daylight type' 6400K (AKA cold light) have two main lines in the green (strongest) and blue wavelenghts, so it would make VC paper soft...
I thought enlarger's ones were a somewhat special.

Now, some practical data:

I've just made two small enlargements of the face of the Sculptor (see gallery).
One was 9s@f5.6; the other one ended at 10.5s@f4 - so, a bit over 1 f stop light reduction using the dimmer.
My reference was the white mark at his forehead, filter grade 3, Polymax II paper.

Yes, there is a diference if you put them side by side - but I would say less than 0.5 grades.
So, I believe that for 0.5 f stops (my intended usage, 1 f stop I get from the lens), it will be insignificant.

Thanks for your comments,

Jorge O