Faceups Burns

I recommend building a little texture with any of the means you can do so for scars, like using a safe glue or acrylic heavy body medium or matte medium over a sealed surface. Seal again with msc or whatever you are using for tooth and then blush with pastels. You can get something very realistic and as subtle (low contrast healed scars) or dramatic and gory as you may want. If you are looking for something more gory you may want to apply gloss varnish where appropriate after.

 

Personally yes, I would seal it to prepare for the Faceup and when that is done I'd create the texture, which will need to completely dry (usually I left it 24 hrs+) then continue the Faceup from there.
Yeah I imagine you can use something like a toothpick. I've heard people say Elmer's but I never used that, I use the acrylic medium only. I tend to build my shapes with a brush because the brush is just the tool I prefer for everything possible.

 

I have few experience with faceups, but I did use Elmer's glue before for some minor scarring on a doll's arm. I mixed powdered chalk pastels into the glue, and I had to do multiple layers to build up the texture because the glue was rather runny (letting each layer dry up prior to applying the next). Result here. And yes, I used a toothpick to build up the texture.

And as already mentioned, totally recommend sealing prior to applying the glue not only to protect the resin, but to better provide the glue some "tooth" to latch on to.

 

So, I'm never shy to look up reference photos of whatever im thinking of, but in general, and the colors of people's scars vary based on their complexion and probably other things too... But in general I base it off my personal experiences.

Spoiler: TW: spiders

for example I have had some severe scars on myself, keloid scars from deep cuts and also I have a large horrible scar from 6+ months of necrosis caused by a brown recluse spider bite.

On myself I notice that my scars which had to heal through damage of all the layers of skin are very dark/bright red at first, like a raspberry, and then they start to lighten over time, going through a greyish purple color and if I'm lucky eventually back to about my skin color.
There is a lot of variation out there on different people.

Spoiler: Tw: friend with burns

I had a friend who was badly burned on a large portion of his body and had received skin grafts and was in a coma for quite a while... But his scars were close to the same color as the rest of him. I think the accident he had was about 3 to 5 years before I saw it.

 

Not sure if this would help, but I have relatively young burn scars and am olive complexion with cool undertones and I lose heat to them rather quickly when cold.

Spoiler: TW Description of Burn Scar Colors and Textures

This makes them appear a little darker purple than what you would traditionally see on paler skin. While I have no burns on my face, the ones that are on my legs where the skin is thinner and closer to the bone look more like giant birth marks (less raised and more of a dark brown skin color with splashes of weird light spots that are even lighter than my normal winter skin) than the ones on my arms (raised with similar patches of light spots, but much more purple).

Also, the degree of burn, the age of the scar, and thickness of skin where the burn took place matter greatly so if you want to complicate things a bit more to fit your character those would be the things to think about.
Good luck with the art! ^-^