If you have a line on the C and the T it is negative.
Thats correct.

If you have a line on only the T it is positive.
Correction. If a line only appears in the C (control) region and not the T (test) region, its a positive.

I have the two lines but the C line is darker than the T line.
Thats a negative. The T line is often lighter than the C line. A non-issue.