It sounds like what Deathrey described. Basically what you would do is create all the lines in photoshop or a similar program, and UV map them onto the field. Then you’d use this mask to make the grass a different color wherever the lines are. You could also use one big decal for this, but if I recall correctly, decals get more expensive as they take up more space on your screen. The field would probably be most of the screen at any given time, so I would put the lines into the field’s material itself.
If you need to go through the basics of materials, I’d check out the official UE4 documentation. It’s a great help when you’re first starting.