On the back of your watercolor paintings, it's a good idea to write little notes. Keep track of what is important to you.
The title you gave the painting, maybe the date or at least the year you painted it.
Do you keep track of your paintings in a notebook with cataloged numbers? Add the number to the back.
Are you new to painting or have trouble remembering what colors you used? Write the names of colors on the reverse. Maybe you have a favorite color combination that shows up in a lot of your paintings. Or you look at one of your paintings and think wow those colors look great, now which ones did I use?
On the back of all my little ACEOs with an india ink pen, I write the title, the year, my name, my websites. Since they're part of my Painting A Day Challenge I add that too, plus the day I painted the little painting.
I have done all of the above from time to time. My larger paintings get pencilled in catalog numbers on the back and sometimes the title too. Before I really learned my palette, I kept track of the colors I used on the backs of paintings.
I use either 140 lb. or 300 lb. watercolor paper when I paint. I have never had any trouble keeping notes on the backs of my paintings in either pencil or india ink pen. The paper is thick enough pen and pencil lines do not show through to the top side. If you're heavy handed and are afraid, lighten up! It's good to keep little notes.