My eyebrows have just begun to grow back in after chemo.... and they started fine and colorless. They are darkening in now, after about one week since growth started.
Certainly, you could use a light colored eyebrow pencil to put a bit of color behind your eyebrows. I'd say that you want to use one shade lighter than your actual eyebrow hair, to make it look natural. I think darker always looks like they are drawn on.... But that's just me. So, if you have dark brown hair, I would recommend a medium brown pencil....and so on.... And use it sparingly.... It only takes a tiny bit to make the eyebrow thinning totally unnoticeable.
I'm probably only at 10% of my original eyebrow hair... and the hairs are super light, but it really is starting to look like I actually have eyebrows. And a wee bit of color behind the hair would certainly make those eyebrows barely different from normal looking. However, when it comes to make-up.... I'm just not in to it... So, I'm just being patient.
Hope that helps.... DOn't worry - they will grow back... and even though it feels like a long time, it really doesn't take all that long....