I have never in my life see or even heard of someone taking a Big Mek with a Burna as an HQ choice. Really, all I have ever seen are Warbosses and Meks with Shokk Attack Guns. I didn't even know you could do this until I started looking for a cheap HQ.
And cheap he is. You can almost field 2 Big Meks for each Warboss you put on the field. (60 vs 35 points respectively). The Mek's stat line is almost 1 below that of the Warboss in every department (-1 WS, -1S, -1T, -1W, -1I, -1A, -1 Ld) so you get what you pay for. They have almost the same access to wargear except that a Mek can take a KFF and SAG. Most importantly, a Mek can take a Burna!
The deadly deadly Burna is perhaps the most versatile weapon in the Ork arsenal. The short of it is that it is a flamer and a AP 3 power weapon but never both at the same time. You get quite a lot for a 20 point upgrade. If the unit gets assaulted, the Burna can be used to inflict d3 hits on the assault unit in Overwatch. The power weapon give a little punch to the squad so that if they are confronted by a Marine army they are not quite as stuck. Not to mention that you could just use it as a flamer and set thing/people/objects/ideas on fire.
The down sides are many as well. It may be AP 3 but then again it is only AP 3. Orks do not have many answers for anything with a 2+ save other than Power Klaws or lots and lots of bullets. If you get stuck in against some termies or a Chapter Master with Artificer Armor, you are hosed.
I also used a Black Orc body from WFB to represent 'Eavy Armor. It is only 5 points and I thought it worth it to protect my HQ. He also got a mask from the Burna sprue because it looks badass, that's why.
That leads me into my next point. If you are relying on the Mek as your heavy hitter in assault, you leave yourself open to challenges. Going back to the example of some squishy Tactical Marines lead by a Chapter Master with Artificer Armor, the Mek would do great by swinging at the Marines or even the Sargent but doubtless the Chapter Master will challenge. Then the Mek is left with the choice of not fighting or getting squished by the Chapter Master. Regardless, it is a winning situation for the Marines.
The Mek has one ability left. On a 4+ he can restore a hull point, repair an Immobilized result or repair a weapon. That's pretty slick. It is difficult to use as he needs to be in base contact (or inside) the vehicle he wants to repair at the start of his shooting phase and he has to skip his shooting. It is nothing to rely on, but it is a nice trick to keep in your back pocket.
Lastly, Meks as an HQ choice unlocks taking a single Deff Dread as a Troop. Nothing flashy as they are not scoring units because they are vehicles. Orks do have some good Heavy choices like AV 14 Battle Wagons and T 7 Big Guns and fill those slots fast. If you found yourself with two Battle Wagons and a big mob of Big Guns and still wanted to take that Dread, for 35 points you could squeeze it into a Troops choice.
For the red/brown and green/patina oxides I used watered down Dark Flesh and the new GW Technical Nihilakh Oxide. The Dark Flesh worked best when applied before the wash to get a two toned effect while the Nihilakh works best when applied thin and after the washes. if you get too much on there, it ruins the effect.