Well, I can name a whole bunch of formulas for smoke compositions, but I've only tried one: The traditional saltpeter and sugar formula. In all likeliness, this is what is used in paintball smoke grenades (I know that many people make their own using this formula). However, it does create a rather large quantity of heat; certainly enough to, say, light some underbrush on fire. Like I said, I can name several compositions from a list that I have, but none of them are called "low heat" or anything like that, and I haven't made or tested any of them.
My suggestion to you is to either:
1) find a way to contain the heat from the saltpeter/sugar reaction (in either a 40/60 or 50/50 ratio by weight [edited] ), or in some other way stop the grenade from starting a fire. I was thinking about this a little while ago, and I came up with the idea to cast the composition into an empty soda can, then wedge the can into a spring from an old sofa. Because the springs are somewhat hyperbolic, they would keep the can at least an inch or two off the ground. I never tested it, but it might work...
2) If you really wanted to, you could try out the formulas I have. However, they call for pretty obscure chemicals, which probably would cost a lot. There's also no guarentee that any would work.
3) look some more. Presumably, since the formation of gas is favored in terms of entropy, there must be a reaction out there that is not significantly exothermic. After all, if entropy is favored there is no need for a very negative delta H number.