implant is a layer used to specify N+ or P+
normally we use a active layer which is considered to be neutral then we cover it with n+/P+ implant layer to make it either N+/P+
in your diagram N-diff is acutually active layer which is surronded by N+ implant to make it N+ doped region.
N well are a seperate layer which is drawn around p+ implant to make a N-WELL in a pmos
Added after 5 minutes:
see attached diagram
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here upper part is Pmos and lower part is nmos.
pmos has a yellow P+ implant , which means pmos then a green colour nwell to show that it is made in nwell.
nmos has sperate nimplant surronding active area.
in both pmos and nmos active area is in middle with same colour (light green)
let me know for further info