An evaporation line is colorless and doesn't have even the faintest of color to it. On the other hand, a positive line has color, usually given pink in most pregnancy sticks. Even if it's a faint pink, it could be a sign of pregnancy because sometimes the hCG level in the urine could still be low.  But the positive line is slightly thicker than the EVAP line, and most people don't even see a difference. Still, the major difference is the presence of color, indicating a positive line.