For example: what color is Kirk's hair in the Original Series? I always thought of it as a dark blond, like this or this. And then I actually watched TOS. Also, I learned that "hair" may not be an accurate term.
Figuring out brown hair is incredibly difficult. There aren't very many options for realistic blond/e hair, and anything dark enough is generally just seen as "black". But brown? Especially for people being filmed under studio lights so they could have several different shades within one episode? Some of whom are wearing wigs/hairpieces that aren't consistent from season to season?
There has been a lot of staring at screenshots. For some characters, I usually start off with an idea of a few colors that might work, then search for screenshots and pick which one works the best. When I got to Data, I realized I didn't have the slightest clue what his hair color actually is. I ended up going with the same color as Riker. Watching the show, I think I did the best possible job with Worf's hair...but his beard is too light. I don't know if it's possible to take out those stitches without damaging the ones around them.
The most recent stuggle: what the hell color is Dr. Crusher's hair? Knowing that she's a redhead doesn't help--her hair seems to vary from auburn to strawberry blonde. Since TNG wasn't kind enough to have an animated series, I'm going with the action figure to spare my sanity.
I should get started on the backstitching even if I want to keep putting off the horrible, horrible metallic floss (and deciding if Data sparkles. Which he so doesn't).