Paint brush? Sure why not.
Brush as in reeds/grass/whatever grows where you hunt? Definitely.

Just bc something is "camo" doesn't make it invisible. Just like deer hunting, you need back drop and to blend with your surroundings.

Best advice is to find an area with tall marsh or reeds that you can hide in.

 

I would make a blind for it. You don't have to go crazy with it, but brown burlap with a couple tufts of raffia breaks up the shape. Color and shape it to match where you are hunting. That is plenty stable to shoot out of.

Here is a how to on what I did for my SOT. You might not need the dog house but use two pole sin the rod holders for the back.
https://archeryrob.wordpress.com/2014/08/17/ride-135-blind-with-the-dog-house/

 

I doubt you have to over do it. I have the same kayak and in the early season I was just pushed in some brush and I was laid back. In full sun I had ducks buzz me so close I could almost touch them. Later, I was pushed against a creek back against the flooded timber, and may most importantly, in the shade. I had ducks sitting down within 30 feet of me. I didn't brush up the kayak but in both instances I was surround by cover that did a good job of breaking up my outline and I was laying back as well. If you are in an area of limited cover to hide in you'll probably need to do more to conceal it.

 

Later in the season especially, you'll NEED to have it grassed. If you can look at it in the marsh and identify it as a boat, the ducks will do the same thing.
Oh, sure, a few ducks will find themselves close enough to get shot, but the majority will spot it and avoid it. They learn real fast once the shooting begins.
The question becomes: do I want in-range shots at 90% of the ducks that come to my spread, or do I want close shots at 10%???

There is NO such thing as being TOO hidden.

 

Ducks usually do a once around and then drop in. They'll easily pick out the boat. If you're lucky and the ducks are stupid, youll get some shooting. But more likely you are going to watch birds flaring away from you out of range.
So plan on grassing/brushing IMHO.