I'm not aware of a way to apply this override on a per-entity basis.
When I do things like this, I always do it the same way:
I place all of the 'existing' things on their own layer, and set that layer to print out light gray or even screened. Then I draw the new stuff next to it, or over it. However, due to how ACADE assigns objects to layers, this isn't always the best workflow.
If there is a need to preserve the layer assignments, then you may want to consider using the 'existing' drawing as an external reference that is attached to the new one. This way, you can change things at will without affecting the original.
This would be done by starting a new drawing, then typing XREF on the command line. (Note that XREF is a vanilla command, but it shouldn't harm any ACADE functionality as far as I know.) Use the resulting dialog to attach the drawing that has the existing stuff in it. Once it's in, you can turn off all unnecessary layers, change their color and other properties, etc. Any changes you make here will NOT be shown in the original file (although there is a workflow for that).
I'm thinking that setting up an XREF is going to be the easiest way to do what's needed here. If you're unfamiliar with the command, review it in HELP. Be aware that this command handle the attaching of many different kinds of files, you'll want to concentrate on the part about attaching drawings.
Hope this helps,
Jim

Jim Seefeldt
Electrical Engineering Technician