In the default Mesh Viewer, the numbers are relative to the home of Z axis (Z=0), the colors go from blue->turquoise->green->yellow->red, where green is for the points close to Z = 0. The size emulates dots following the perspective of an observer over the bed, big dots are close to the observer, small dots and far from the observer. If you create a mesh before printing in your start g-code, you can close the mesh viewer by writing the custom g-code C108 in a line after the G29 (G29 P1).