If you want super easy, it's hard to beat asking HomeDepot for their least expensive tintable Glidden, interior, flat, and having them tint it "Universal Grey".
If you want something both a little darker-colored and a little higher-gain, HomeDepot sells PPG tintable silver metallic for $22/quart. If you buy a quart of that metallic (have them shake it, but don't add any tint to the metallic) you can mix 16oz of the metallic with 4oz of flat Glidden paint that's tinted a darker grey like "Granite Grey".
It can help in this case to first use 16oz of the dark-colored Glidden and combine it IN A SEPARATE CONTAINER with 5oz-6oz water to give the panel a really thin and light coating of flat paint. This will look like crap on its own, but gives the metallic mix something smooth but easier to stick to once it's dried for a few hours.
Once the Glidden+water single coat is dry, you can mix your 16oz PPG metallic and 4oz Glidden grey together in a container and carefully roll it on with little/no pressure and only rolling in one direction while using a new 1/4" roller-cover (they sell 3packs at Wal-Mart for ~$4 which is much less than HomeDepot).
If you're rolling the metallic mix while the panel is laying flat on the floor, you can thin the mix a bit with 3oz-4oz water, but if you're painting the panel while it's upright I don't really suggest thinning the metallic mix...maybe 1oz.
Here's a video of how I've been rolling metallic. Pretend this section is 7ft wide instead of 2.5ft wide and still 4ft tall...a short 2ft roller want/broom handle can help if you aren't comfortable reaching/leaning over a 4ft tall panel to paint it.
I have longer written "instructions" also explaining this rolling in a bit more detail if this isn't clear..and/or feel free to ask any questions you want.
This isn't the only way, but it's a way that's worked well for me.