If my answer below is not helpful, I strongly suggest you post this question in the 60-66 forum. My original reply was to help find the paint formula code. I have no first-hand idea where to find the Trim code or Paint code on a 1961 Chevrolet C20 truck.

However, do you have a Shop Manual for your truck? My guess is that it would be a 1960 Chevrolet Truck Shop Manual and a 1961 Truck Shop Manual Supplement.

In the 1960 Manual, on page 1-5 is an illustration of the Vehicle Identification Plate, which is installed on the left inner cowl. That is: the side panel above the floorboard on the driver's side of the cab (I'll post a photo of the location, if you want). The Trim Code and Paint code are near the bottom of that plate (I'll post a photo of the plate, if you want).