after adding a graft such as a genital graft. Genesis 8 figure, the graft is grey, so how do you get it to be the texture of the figure?

The default G8  figures in the starter essentials do not have mat presets or textures for the grafts.  You would need to use a figure that comes with the proper texture files like V8 or M8.  Other PA's may include them with their creations, and most of the time it would be mentioned in their product description

Here are some instructions that I have cribbed from the documentation of a product that's sold on the naughty store, this might help you out:

1.(If your character does not have a genital texture)
Load your character then load the genital prop which has no texture. Select the Genesis 8 female
figure in the scene tab then in the surfaces tab select Surfaces/Torso and right click then copy selected surfaces.
Select the Genesis 8 Female Genitalia then in the surfaces tab select Genitalia, right click and paste to selected surface.
Now still in the surfaces tab select Genitalia/Geometry and in the UV Set parameter drop down, change the
Default UVs to G8F Torso2Gens
(NOTE: There is sometimes an issue with Iray shaders when copying the torso textures to the genital prop
which means the correct values do not get copied. This is because some vendors set some shader parameters over limits.
If this occurs (you will see a mismatch between the torso skin and the genital skin in an Iray render) then the only solution
was to manually change the setting of each effected shader parameter to no limits then set the correct value by comparing
with the torso texture. It was a bit of a pain and took some time However, to save you that time I have made a material
preset of the default G8F torso texture that has all limits switched off which means that any texture/shader that is subsequently
pasted over that prop will copy correctly even if some parameters are over limits..
Find it in People\Genesis 8 Female\Anatomy\Eroticalist\G8F Torso Texture Iray No Limits. Select the genital prop then load
this material preset onto it first, then paste the torso texture of your chosen character on top. That should do it but sometimes
one setting stubbornly doesn’t copy correctly so check the Thin Walled switch found in Volume/General of the surface
parameters is set the same as the Genesis 8 female torso texture, OFF or ON. Not an ideal situation I know, it seems to me
like a Daz bug and took me hours to find out what was happening here but this fix does work.

Oh, not the naughty store,

thank all for this info, and now back to work. but the naughty store hee hee!