Clearblue Ovulation Kit

Melissa
Hi! 
I started using OPK tonight. I noticed in the directions it says that the color on the strip should not be used to determine anything. Only use the test reader and that the color of the test line doesn't matter.  Is this true? 
According to tracking I should be entering my fertile window... the test was negative but the line was quite dark compared to the control line. 
Thanks! :)