I usually dabble with movie-based creations, so when I first build it, it's usually (Not always, though) from memory. Then, I typically rework it a couple times, and sometimes, I'll sketch out an idea I think is good (Take my 8-stud Ecto-1, that was sketched). I usually tend to like my sketched ones better than other ones.
I used to never care about coloration, even with the final project. That's why my first Delorean was red and dark grey with green doors. Now though, I almost always build with the correct color from the start, though not always. My Sweeney Todd MOC, for example, had tan walls and a yellow door, only because I couldn't produce the correct colors- Sand green walls and a brown door.
The only times I've built a creation on LDD is if there's no chance I could reproduce it in real life. So far, I've built Stay-Puft, a Tron Recognizer, Jurassic Park gates, and a pet project of mine (Which shall rename nameless for now
on LDD. All of those were very large MOCs which I couldn't produce in real life with my creation.