Yeah...I think you're describing either a "dashcoat" or "sand" finish. Whether stucco or Dryvit, keep one thing in mind--In fact, say it out loud to the customer; "A patch is a patch is a patch is a patch ia a..."
I've never seen a Dryvit patch blend--EVER--No matter if I did the repair or anyone else. Drivit is a "Get it right the first time" kind of material...
Especially a dashcoat finish...
Do what you can to find the manufacturer and product, ask the customer if there's a storeroom where leftovers from construction were stored. If it's a synthetic stucco you're going to be best off with the original can of sauce, especially considering color...
After you've done all that--------RUN FOREST! RUN!!!!