I have a Blue Stripe Coliseum and a "100 watt" Blue Stripe, and with the same settings, I don't hear any difference between the brightness of treble response.
What I do notice is that the Coliseum has a ton more balls that the normal Blue Stripe. The most noticeable qualities of the Coliseum is that it sounds a LOT thicker than the normal Blue Stripe, and that the feel is much quicker. IIRC, (and feel free to correct me with photos those of you that have IIC+ coliseums) the Mark III Coliseums had the same OTs as the Mark IIC+ Coliseums, and if you've studied up on your IIC+ knowledge, those amps are known for their responsiveness and feel.
Another fun fact is that after the Mark III Coliseums were discontinued, those same OTs were used in the Bass 400+.
Ok, back to the low-end characteristics, the Blue Coli can be made to have the same type of low end as my Rectifiers, and it may have very well been the jumping off point where Mesa decided to split the Mark and Rectifier lines (there's a few threads about this a few years back, so check them out too), but I think the Rectos sound much more 3D, whereas the Coliseums sound a little more dry. That's a long way of saying that if you're familiar with the "girth" of the Rectifier's low mids, the Blue Stripe Coliseums have that tight low mid where the normal Blue Stripe just stops at a certain point.
Don't get me wrong, any of the Mark IIIs are a seriously killer and underrated amp in my opinion, but the Coliseums are more than just a high output amp. They have their own extended voicing that rather than getting brighter, they have more depth.
My No-Stripe Coliseum doesn't quite have the same low end response as my Blue Stripe Coli, but still sounds thicker and more responsive.