Question
why would these line that run from septic tank grass turn brown.
most yards are a darker green color?

Answer
Hi Carol, two reasons I know of. One, some septic additives (root killers) etc. may be toxic to the grass.  Two, the ground is staying wet and fungus has attacked the grass.  Try a good lawn fungicide, check your pH and adjust it to 6 - 6.5 if necessary and re-seed the area.  You may want to switch grass types to see if another variety is less prone to damage.  Jim