A colour enlarger will also have a 'white light lever' which you can move so that the image is brighter to aid focussing, It will be on the side of the head. Find this lever and move it to the opposite end of the arc to where it is now and check the brightness on the base board. If it darkens then that is your problem.

I use an LPL enlarger which is not all that different and found when I got it there had been little use for a very long time and the colour filters in the head were sticking this may be what is happening with yours, however I would go for the white light lever first and see if that works.