THE MENU MUSTARD

... is correct about the liquids. The two factors to a mustard's heat are a) the type of mustard used and b) the liquids mixed with it. Yellow mustard seeds are the mildest with brown and black seeds having far more heat (your Coleman's is a blend of yellow and brown seeds so there is a good amount of heat there). There are enzymes in the seeds that are dormant until activated with a liquid. These enzymes release the pungent/spice compounds. Acidic liquids (vinegar) slow that release down. With water, cold quickly activates them while hot water deactivates the enzymes and breaks down the compounds. Added heat (cooking the mustard) further mellows the final product.

So if you want a mild mustard, use yellow mustard seeds, more vinegar than water, hot (boiling) water and gently cook it. If you want hot mustard, darker seeds, less vinegar and cold water.
On a side note, regardless of the method, all mustard has a bitterness when the seeds are first cracked open. However, the bitterness diminishes quickly and by day three is usually gone completely.
Soaking the seeds a few days prior to blending in your choice of vinegar is a great way to go.