Well, I painted my deck almost 20 years ago, I used tempera and was quite fast, too, but my drawings were much more simple then yours.
But I'm not at all a professional painter and I never used watercolor...
well, I've been an interior designer for many years, I know there's a very little difference in price between a B&W or watercolored reprint of Piranesi
https://www.google.it/search?q=stam...rOIKj4gTFpYHIAQ&ved=0CDIQsAQ&biw=1431&bih=843
I've seen people painting these B&W prints - they are often students at the Academy of Fine Arts - and yes, they need only few minutes, more likely they are faster then me with the digital color filler.
More seriously, I think we are looking the problem from the wrong side.
IMHO, a professional painter wouldn't have used watercolor for a painting needing
"to stay within the lines and keep the color smooth"
But, actually, did PCS needed to stay exact whitin the lines and keep the color smooth?
I'd say no: there was no photocolor at that time, there was no need of this level of precision, as someone else will prepare the printing plates anyway, using the B&W as a reference.
Of course I have no evidence, but, admitted that PCS did the coloring and did use watercolor, I imagine the original had a look more similar to some Hugo Pratt watercolor then to the solid color version
https://www.google.it/search?q=hugo...fe_rd=cr&ei=VmfyU-3XNIze8ge2-oGYAw&gws_rd=ssl