Faded means the row is finished. You've marked every box, you can now go ahead and X the others.
If you make a mistake and render a row unsolvable, the entire thing will turn black, including numbers that were previously grey. Note however that it is possible to make a small mistake that does not render the row unsolvable immediately, but will do so later down the line. There's no immediate way to detect this, but if you're being careful it shouldn't happen often.