How do I touch up the trim of my home?

Make a paint chip. I have used a razor knife in the past to cut out a small thin section of painted trim and brought it to the store for matching.

Get a paint swatch ring of the brand you like; most hardware store will lend one to you with a deposit. Refunded on return of the swatch ring.

If you aren't near a store that does that then pick up swatches that you think are close and then match them up to the trim piece. Once you have the best match get it and give it a try on a small area. When it dries you'll know if it matches. If it doesn't quite match go back to the paint store and have them add what it needs. (It can be adjusted a little lighter/darker/browner, etc.

Its not hard, but does take some back and forth. Three trips to the hardware store for every project seems to be my motto...

Take a photo with your phone if that is an option for you. Head over to the paint store and buy a few sample sizes and test them.

Photograph it or better still match the colour with anything available (Fabric/Wallpaper/Paint card and get it made up for you at Paint shop or DIY.

You cut out a chip in an inconspicuous spot and have the paint department/store color match it. A photo may be a problem. Not all colors are exact/depicted in a photo.

I would use a utility knife to get a chip of paint off of the trim. The paint store needs a piece at least the size of a nickel to be able to get a true match.