It's not a silly question.
Yes, the blonde shade will color the greys blonde, and will just add some shine to the rest of the hair. It's a non-lifting color so it won't change the rest of your hair. Most of the RC Swatches are done on white or blonde hair, same as other brands. That's why they come out darker if you have darker hair. So if you choose a blonde shade, it should turn those grays into highlights for you.
If you tell me which RC swatch is closest to your natural color, and what color you'd like the highlights (grey) to be, I can help you choose a color. LMK