I once accidentally changed my crosshair to a green cross from the default. I wanted to change it back, but after really thinking about it, I realized I was so used to the default crosshair that it became totally transparent to me. I couldn't even recall what it looked like. I had to go back to an old screenshot to figure out the settings I used.
For me I'm comfortable with a crosshair that isn't too prominent. The default adaptive white/grey crosshair does it for me.