Tie Dye and Hand Dyed Thread

I love using hand dyed thread for tatting and always wanted to try my hand at it, so when my daughter asked to do tie-dye, I thought this would be the perfect time to dye some thread.  We went off to by some Rit dye and white t-shirts and bandannas.  As you can see below, the tie dye turned out beautifully.

The colors are bright and the tie patterns turned out really well.

The thread, on the other hand, didn't turn out so well.

See how wimpy the colors are.  The one on the left is supposed to be purple, the same color as the big t-shirt above.  The middle one is variegated blue and fuchsia, and the one on the right is supposed to be royal blue.  I dyed the thread the same way I dyed the t-shirts: 3 gallons hot water to one bottle of dye, 1 cup salt, soak for 30-60 minutes.  Why did the thread not color as well?  I'm almost positive it's cotton (the label is missing, but it feels like cotton).  The dye seemed to rinse right out of the tread when I took it out.  The purple is a little darker because I didn't rinse it right away.  I let it sit for 20-30 minutes after taking it out of the dye bath.  Can anybody out there tell me what I'm doing wrong?  Is it the dye? the thread?  Should I add vinegar instead of salt?  Please help.  I would love to dye my own thread.

On the tatting front, I did manage to get one small motif done this week.

This is from the book Butterflies Migrating by the Palmetto Tatters.  It is done in Lizbeth Pink Parade and Tropical Punch, leftovers on shuttles from previous projects.  It ruffles quite a bit.  Maybe the picots on the inner rows are a little too big.  But I like the shape.