MeshStyle is an option for Plot3D, DensityPlot and related functions which specifies how mesh lines should be rendered.
MeshStyle can be set to a list of graphics directives including Dashing, Thickness, GrayLevel, Hue and RGBColor.
See the Mathematica book: Section 2.9.3.
See also: Mesh, AxesStyle, Prolog, Epilog, DisplayFunction.
The second plot colors the mesh lines.
Evaluate the cells to compare the graphics.