I had some clear Waterguard exterior deck stain/sealer which is water-based. I got them a while ago so I cannot return them. The problem is I like to cover the deck with opaque color. Is it possible and a good idea to mix the Waterguard sealer with Behr water-based wall paint?
First, are the products chemically compatible? The mere fact that they're water-based does not make it so. The best place to ask is the factory.
However, there's a bigger problem. Even if they're compatible, a colored paint diluted (what, 2:1?) with a clear coating will have very poor coverage. If it's going to take 6 coats to get decent coverage on the deck, then why not just paint 4 coats of clear stain, and 2 coats of paint?
The third problem is that floor paint is a very tough service. I don't have confidence in any water-based product, but you certainly don't want to use wall paint! For best use of the materials, you might try 1 coat of sealer (to actually seal), 2 coats of wall paint (to color), then more coats of sealer (to provide a durable coating over the color). That's a stupid amount of work, and I still wouldn't have high hopes.
If it goes wrong, you may have to remove the failed paint. That will be far more tedious and expensive than any savings you might have had on paint.
Believe me, I get it. Nobody wants to waste paint. I have tens of gallons of paint I have no use for (I am a fastidious color matcher, and I buy a lot of quarts just for color tests). But my problem is, everything I might paint is too valuable to risk a failed paint job and the huge amount of work to recover from that.
There's one more problem with newer water-based paints. The paint industry has done their level best to reduce VOCs, especially toxic VOCs. The result is nothing poisons the paint from growing microbes or mold. There have been many cases of people finding paint that's stinkier than normal, painting their walls, and finding the stink won't go away. Covering it with fresh paint doesn't work, their only option is to peel off all the layers of paint down to the failed coat. This is a staggering amount of work that can go into hundreds of hours. That makes me very reluctant to use old paint at all. Again, most of my projects are too important for that kind of risk, so I find myself just buying new to avoid any chance of a big problem.
you can add pigment to the paint ("sealer"). Oxide pigments are sold for tinting cement but also work on paint. In fact this is the same stuff paint manufacturers use to colour their paint.
Or you could take it to a paint shop and ask them to tint it for you... a clear paint will need a lot of tint to make it opaque and will be diluted by the liquid part of the liquid tinting compound used in retail, and you'll probably have to pay for the service.