Red is for cutting. Blue is for etching and black is for rastering.
Rastering is lasercutting dot by dot, and this will help to speed up your file and provide a cleaner line. If you want a thicker text line like some of the other texts, leave it greyscale.
If you are using Corel Draw the red and blue lines should be hairline. For Illustrator they should be .001 pt. Black raster lines can be any size. In addition you should be working in a RGB file.