In the
Walmart party section, they have single-color packages of Kisses, etc. that might be fun for a colorful rainbow look. They have candy sorted by color at Party City, as well. Walmart also sells big bags of assorted candy--you could dump those in a bowl, or separate them out. Candy for 70 people is NOT going to be cheap, however you do it.
Keep in mind that it's so hot out--chocolate might not do well outside, this time of year.