In numbering the problems on pictures of various boulders, I've tried to find a system that would give a quick visual reference for someone looking for a particular difficulty level. I first encountered a system like this while bouldering in Las Vegas, where the guidebook I used had the problems marked in circles of different colors, and I could quickly flip through the pages looking for "green" and "blue" problems that would make good warmups. In trying to adopt a similar system, it occurred to me that I know at least three climbers who are colorblind, and I decided to use different symbols in addition to different colors. The resulting system is a bit cumbersome, and I'm still not entirely happy with it, but here is how it works for now:
Below is a picture of these symbols in use. Each of the four problems is indicated by the shape and color appropriate to the rating. For example, problem #1 is marked with a blue diamond, indicating that it is within the range of V6-V7. Each problem starts approximately where the bottom end of the line is, rather than at the symbol itself. You'll also notice dotted lines in some pictures. Problem #2 in this picture is shown as a dotted line to indicate that it diverges from the main line until rejoining it at the slot.
In the case of the picture below, problem #3 is marked as a dotted line to eliminate confusion about where it goes when it crosses the lines of problems #1 and #2.
Like I said, it's not a perfect system, and I'll happily take suggestions about how to make it more user-friendly. Any changes to the system will also be reflected on this page.