I've seen DIY heat pads using white Christmas lights. These are on clearance but are multi color? Would they work for seeds?

Maybe not, if they are LED instead of incandescent bulbs. The color would not matter, but if they are LEDs, they might not draw enough power to produce much heat (not enough watts).

If the article that taught how to make DIY heating pads / boxes said how many watts are needed, you could compare the wattage on the package to that number.

I didn't see "Watts" or "equivalent watts" or "LED" or "incandescent" in the photos. If they are LEDs and the wattage is only specified as "equivalent", they mean the total brightness is comparable to an incandescent string drawing that many Watts of power. Ignore "equivalent Watts".

"Real" seed heat mats have wattages as low as 17 W, if they're for just one tray. Do the "tree light heat box" plans call for a thermostat to keep the soil from getting TOO hot?

I looked on the boxes & couldn't find LED anywhere either. The plans I saw were basically lights running around wood with a shelf thing that the seed containers set on, keeping them from direct contact with the lights. I decided against getting them.

I guess there's no way to test them for warmth at the store.

I played with the idea of using that heating tape they wrap around water pipes to keep them from freezing - but they are expensive.

Here's one that uses incandescent rope lights. The article stress to use incandescent not LED.
http://www.vegetablegardener.c...

The first box does state "traditional lighting" which likely means incandescent. You should be able to look at the bulbs closely and tell the difference since the inner workings look different. Still, I'd be worried about them getting accidentally submersed in water at some point under a seed tray. They are made for outside use and rain, but still. It's probably just the klutz in me that's worried about accidentally dropping a gallon of water on them.

Me, too. 120 Volts AC plus a watering can ... what could go wrong?

I was thinking that if I made one of those, the water barriers would be elaborate and gadget-y. Like everything else I fiddle with!

"elaborate and gadget-y"
Yep, next thing you know, Rick will be exploring the Mariana Trench in his homemade seedling heat-mat!

And it would stay dry!!

But it might take a crew of five to operate it.