How can I color concrete?
After using the similar forms you describe, poured about 60 feet of walkway -- I've used the Bear concrete stains which are applied after the product cures with great success.  Even taken two different shades and did the "artistic" thing providing a little depth and 10 years later still looks good -- although an extra step, it went quite quickly.  Using the "artistic" approach allowed us to use the $5.00 mis-mixes with a little creativity.

hope it helps...