Do darker beers ferment slower?

I brewed what was supposed to be a dry stout 3 weeks ago. It started at 1.043 and is only down to 1.012 now and it tastes sweet.
I brewed a pale ale 2 weeks ago that started at 1.040 and is now down to 1.004.
Both were fermented in the same room pretty cool...the low 60sF, with nottingham yeast. Do darker malts ferment more slowly? Do you think my stout is stuck and should I pitch more yeast or do something else? I decided to hook up a warmer and warm all my beers up to 72F or so, but I'm worried my dry stout is not going to be dry.

no, color has very little to do with fermentation.

Yeast don't care what color the wort is. They only care about food (sugar) and whether they can eat it.

Your darker beer has more unfermentable sugars that are extracted from the darker malts so you probably will not get it to ferment down to less than you now have it. It will change flavor when it is carbonated.

If it stays too sweet, could I add some enzyme or some hop tea or something to make it more bitter?

Well, I warmed it up to 21C, dumped a another packet of Nottingham in it and gave it a swirl. I'll give it another week or two and check it again.

It never hurts to wait longer and see what happens. 1.012 is not unusual for a dry stout FG. Don't be surprised if it doesn't drop more.