To make a roof is simple - create a "scaffold" of up/down stairs along the outside wall of the building leading to the roof, then order it filled in one ring at a time, building up ramps if you want a sloped roof. I suggest setting up stockpiles for materials with wheelbarrows to ship material from one stockpile near the masons to a stockpile right next to your tower, or maybe even a minecart path if you will see enough use or can reuse enough of the track for more towers.
That said, when he says "plan out colors beforehand", he's not kidding. When LoudWhispers made Silentthunders, he wound up having to strip-mine essentially the entire top two layers of stone in his fortress to keep it all "white stone" because he started building with limestone and marble.
The only infinitely renewable materials are wood, glass (in green or clear when combined with extreme wood farming), ceramics (earthen or stoneware bricks), and obsidian. Maybe weapons-grade metal if you're absolutely masochisticly determined to abuse exploits beyond all reason and want to set up industrial-scale bolt splitters and melting systems. (And to be fair, it IS pretty metal if you tried to make an entire fortress out of steel.) Beyond that, you'd have to set up cheat reactions to go beyond your natural excavation limits.
If you want to use one type of natural stone, it's probably best to go for whatever is on the bottom layer of your embark rather than the top, since you typically find the last layer goes down around 60zs rather than the 7 or so zs the top layer goes down. Remember: You get only 1/4th the boulders from digging stone, and can transform them into 4 blocks, so you're basically digging out as much space as you're building. 7 zs of material, even clear-cut, only gives you 7zs of above-ground space.
If you want to use obsidian, however, there's of course the significant advantage of it being "natural" stone when you cast it, allowing serious megaproject builders to create "cranes" that pour magma and water into moulds made of "scaffolding" walls, then carving their fortress out of the result. This further allows for engraving above-ground "constructions", and also creates a single black, shiny obelisk of dwarven might to present to the world. Oh, and making the roof won't be a problem, either.