Well it is important to try to LOOK as much like your modeling compcard as possible, but if you happen to change your hair color don't fret!
You might not need to change your photos 100%. But you do want to LOOK like your photos -you don't want to show your hair blond and then show up brunette.
But no matter your hair changes I would say KEEP all your photos. For example, my hair has gone from really boy short to really to my boobs long. So all of these lengths have had different color of hair, shade or hair, as well. Some show high lights, some show dark, dark hair. I would say ALL of your photos should be kept, not used ALL at once but always have your photos old and new handy because..
For example: If a casting wants a short hair girl, you could still be submitted because YOU HAD Short hair before, and you could use a wig if you book the job.
Also if you are willing to cut your hair again, that might help you book the job too.
You might want to show what you hair looks like in many styles...
To your agency
And when comes to deciding on your look and being available for all types of looks.
Also as a model your hair is bound to change. It is normal, but here are some No's for the commercial print world.
Short and long hair of all lengths is cool, but if you have many colors in your hair, that is NO. Unless it is for a hair care company that shows funky edgy hair styles in their ads. Normally for commercial print modeling you want a simple color, one shade, all around the head!
Look at ads, notice commercials, look at what is out there, it is a good way to know WHAT WORKS. Most girls have a solid hair color.