Hi Mike,
As Bill stated, paint stores (at least none I have tried) can make a paint directly from RGB numbers. In theory it should be possible, but that is not how their color-matching software works. There is a great amount of data that gets used by that software that could be reported, but the software manufacturer (usually X-rite) just gives the info that they think the store needs to mix the paint and nothing more. I'll have to admit it's a bit frustrating at times for us. Sometimes at Lowe's I will remember to ask what the "Delta E" (the color difference between the color you are having matched and what the computer says it will make) is of the match when I have a color matched by them; about half the time the person mixing the paint doesn't even know that info is on the screen.
About your photographic gray card; if you mean a Kodak Gray Card for use with light meters, they are 18% reflective and are an N5 gray value in the Munsell Color System. The RGB is 119, 119, 119.
The RGB values of 185 would actually be a N7.5 value. Of course, your card could actually be this value if it is just for use as a white balance reference (the WhiBal card comes to mind). With todays digital cameras and light meters I guess there isn't much call for the old Kodak Gray Cards any more.