Is there a way to Touch up old exterior paint so it will blend?

You would have to color match the faded old paint, get paint chips and match, or bring a piece of siding to the paint store.

It's really difficult to match indoor or outdoor paint after it's been on for a while. Sun and rain fades paint and certain chemicals in the air affect the pigment. What I've done is gather a lot of paint swatches, choose one as close in color as you can, then add light or dark paint until it matches.

If you bring in a sample to the home improvement store in your area, they will try to match it for you.

Faded paint is very difficult to match

I agree with all of the above comments. You might have a hard time matching the paint. If you do, I recommend going to a real paint store like Sherwin Williams. If still not happy with the matching, you might have to paint that one side.

I agree with Sharon. This worked for matching my siding. HOWEVER, I found that if I used the exact same sheen, the new paint was shinier, because it wasn't weathered, so keep that in mind. I have spots that never blended in, in terms of shine, but the colour is perfect.