Celery needs lots of water and fairly fertile soil. Also, store-bought gets covered with something to make it grow pale. I love home-grown celery, especially for cooking. But,mine is also deep green and tougher. I grow it during winter in a shady spot because it can get too hot for it even during winter in AZ. I also let it go to flower at the end of the season. It really attracts the good bugs and even had a bunch self-sow for the next cool season!

I've grown both those varieties here in western OR and they should get full sized and be similar to what you find in the store. Tracydr is right about the blanched stalks (which is easy to do) making them lighter colored and more tender, but your plants should be getting much bigger than you describe. So my thoughts are that you're not giving the plants what they need, (which as tracydr also said) is lots of fertility and very regular watering.
Celery is possibly the hardest veg for me to grow and i've given up growing enough for the farmers market I sell at. Instead i'm opting for more celeriac, which seems less picky.
I suggest you keep building your soil and do some extra research on growing celery and it's special requirements, and you'll get it right. Good luck.
-Mark

In our part of the country, celery is almost exclusively grown in muck soil. I'm not really sure how you would simulate muck - it's sandy, organically rich (black), and floats on an endless supply of water. The link is a very short read, but interesting nonetheless.

Here is a link that might be useful: Celeryville