If the stock is pre-oiled and I wanted to match another stock, I'd do as suggested already...but I'd do it on the inside surfaces as a test run to be sure. Tung oil doesn't change the color or really darken it much immediately, it takes time as it oxidizes. The oxidation takes on a yellowing effect, and when combined with the tannins in the wood, and other environmental factors such as handling, oils, dirt , etc. Will get that yellowing to a darker reddish. UV will generally bleach out the color. That's my understanding anyhow.