I usually don't have anything left except for small amount of sand/portland stuck to the sides. I wash out my mortar pan in my yard, been doing that for years, I have a nice lawn from the lime. If you're using a gravel mix (any mix for that matter) and have leftovers keep small molds handy and make pot feet.
Any environmental issues with dye would be addressed on Quikrete's web site. Billie

Same here

It depends, of course, of the quantities. I can't promise you that spilling 5 gallons over a small location wont kill your lawn, but I too wash my containers over my lawn after working. Doing so has had no effect on my grass whatsoever.

Now, depending on how much you have, you could also post a giveaway. A lot of people here use a lot of dye. You could offer it if they pay for the delivery... just an idea

Ok, but what type of environmental effects are we talking about? Do these dyes break down in the soil? I'm a bit what you would call organic, and I don't want to pour something out that I know will be in the soil a few years later if I decide to plant there...

From Quikrete's web site, MSDS sheet on dyes says "hazardous decomposition or by-products: none"