Make a froe from normal mild steel, say 1/2" thick ?

[ThomasPowers]

Yes, No, Maybe.  Forges burning charcoal have been in use for over 3000 years and used for things like forging Japanese and Viking era swords. Not being able to see your forge makes it had to tell if the design is good for using charcoal.  (Just like a gasoline engine differs from a diesel engine; a forge's design needs to be correct for it's fuel.)

Now leaf spring *IS* much harder to forge than mild steel so that can be an issue too.  As can be the size of the anvil, weight of the hammer, temperature of the steel.   Temperature of the anvil/hammer as well and how much experience the smith has.

(Coal started to be used by smiths in the west in the high to late Middle Ages, "Cathedral Forge and Waterwheel", Gies & Gies; but charcoal has never stopped being used.)

[anvil]

Its really hard to tell colors when outside. A dark box can help. this can be as simple as a 5 gal bucket kept near your forge. put your iron in the bucket, out of the sun, and you will see the colors in a constant light. compare the two colors, time of day, etc and you can be pretty close with a little experience. Lol, otherwise if its sparking its too hot, and if it is hard to forge, its too cold. Until you are familiar with colors in any condition, the only thing to say is leaf spring is harder to forge than mild steel and your leaf spring is pretty big, so its tough working anyway.

wood chunks and "cowboy Charcoal" is not the best fuel. You are from Grand Junction. You are near a number of places to get inexpensive blacksmithing coal from the mine. You are also within a couple hours drive of many good smiths. Find them and ask about coal. I'm in Mancos, a few hours from you. Hesperus has good coal.