I work in the past.  Still on AutoCAD LT 2006.  I do not do extensive, detailed drafting.  I am being contacted by engineers now to develop plans for them with their site plans (civil site plans).  They send me their base plans for me to develop erosion and sediment control plans and then send them back for them to include in their plan set for bidding.  They have their system set up to plot colors a certain way and the layers use all colors.  My way of developing my plans, I need the existing conditions to plot light gray, the proposed to plot slightly darker, and my plans to plot black to stand out.  How do I set his different parts of his plans plot gray and mine black without changing the layer colors of his plans and set them in plot styles because when I send it back to him, it loses everything I have set up.

Create a custom plot style that reflects the way you want things to look once plotted.

I understand that I need to create a new plot style but here is the problem that I can't figure out.  His base drawing uses all colors basically.  My plans I need to plot his existing contours as light gray, the new infrastructure as dark gray, and my additions in black.  For the existing things I need to plot light gray, he uses some of the same colors as he uses in new infrastructure so if I understand plot styles correctly, if I set say green that he uses in existing to plot light gray, that will also make green that he uses in new infrastructure to plot light gray when I need it to plot dark gray.  How do I do this?

You would probably have to look at using a name based plot style (STB) instead of a color based plot style (CTB).

What about selecting all of the existing and changing it to one colour, the proposed a second colour and all your new drawing a third colour and set up your plot styles accordingly.