Based on my experience with colour neg printing and the Durst M605, something is seriously amiss(and yes I know you have in effect told us that) but it may be some comfort to state that I have never had such a problem so if everything else is OK such a problem shouldn't exist therefore can be solved.

I'll ask all the usual questions first: Do the negs look particulary dark when you hold them up to the light?

You don't say whether it is a 35mm or 120 but are you using the right lens with each(50mm/80mm). Even the "wrong" lens should give plenty of light however at f4 and the 80mm should go down to f4 or f5.6 at its smallest.

The light intensity should improve by leaps and bounds as you move the lens through its range of aperture, does it improve at all. If not could it be stuck at f22 even through you are apparently moving it? Take it out and try it, holding it up to the light.

There is a light box that moves from 35mm to 66. Is the lever fully home at both settings. If it sticks in the middle then ot can shade the light from the baseboard a lot. Have a look inside the box and move the lever both ways, does the two settings give full exposure by moving the acrylic vanes fully out of the way?

Is the bulb sitting square in its holder? Bulbs in the 605 are a bit fiddly to set correctly. It is easy to place the bulb at an angle. While looking does the bulb and reflector look OK. The bulb should produce such a bright light that you can't look at it for more than a second or so without being blinded.

You don't say whether you had any problems with B&W negs prior to attempting to focus a colour neg. If B&W negs were brightly illuminated at big apertures like f2.8, f4 or f5.6 then either something has happened by coincidence to one of the items mentioned above when you've switched to a colour neg or we're back to the look of the colour neg and it's very dark which suggests a colour processing problem. Have you got any other negs to compare with?

All you can do is work through each possible problem to see where it leads you but rest assured a bright neg projection is the norm.

Best of luck