I've heard a lot of people say "buck brush" but I've never known exactly what it means. I'm sure I've fished it a lot, just don't know what they're referring to.

This.

The thick, approx 2-4ft tall bank lined bushes with a good solid wood base. Fishies really like these bushes certain times of the year. They nose up to them and talk to the wood.

So basically dead small trees? Would dead, flooded willow trees be considered buck brush?

Also, any tips for fishing this stuff?

Heavy t-tigged creature bait or jig pitched in there as close as possible. 50lb or 65lb braid and be ready to hoist 'em out as quick as possible.

I also like to skip a fluke under the branches to get way back in there, but the further in you go- the harder it is to get the fish out.

You fish it with a stout rod with good backbone, strong line, good, sharp heavy hook and eyes wide open. When they bite you better get them coming your way as soon as you set the hook.

One of the most fun ways to fish.

Buck brush is the common name for it. It's a species of shrub that grows along the shoreline. Willows are an entirely different tree

Throw jig or T-rig into the middle of the nastiest one you can find and hold on!
worry about getting the fish out after you hook him.LOL

Any recommendations on colors? I'll be fishing either Granbury, Weatherford, or Mineral Wells. What works good in these overcast conditions?

Buttonbush, wax myrtle, etc. Flooded bushes with woody stems. Spinnerbaits, TX rigs, slip jigs or flippin' jigs tend to be the best tools for tackling "buckbrush".

You guys left off one very effective way to catch them outta buck brush. Throwing a Frog on 65-80# braid is an awesome way to catch them when they are back in the buck brush. I like to start with a Stanley Ribbit Frog on a Double-Take hook. If you have any that swirl on it but don't commit, throw a hollow body frog back in there and slowly walk it out.

Try not to jerk it away from them when they blow up on it like Dustin & Jake....

The farther East you go the more actual Buck Brush you will find. They stay green even when the water is up on them. They love the water and keep their leaves year round. It provides the much sought after shade. I have won many tournaments flipping a spinner bait on Ky. Lake and the TN. River back in the day out of Buck Brush.

1/2oz Mpack flipping jig with a xcite raptor tail chunk trailer
Heavy action rod with 65lb braid or 20-25lb floro

No its the stuff in the pic above its not dead.