It's actually super easy, you just need to make a new image 16x16 pixels or multiples of 16 like 32x32 or 64x64 or you can do 16x64 etc. Then you just use the color chart in petz and paint whatever texture you want using those 256 colors. Then save it as a .bmp and put it in game and see how it looks!
Or a little more complicated way is to take an existing picture and then convert it to a .bmp, but converting it skews the colors, and even then how it looks is not how it looks in the game, because the game will just show whatever color it thinks is closest color to the picture. I have yet to ever get one to work for me this way, they all turn out ugly lol.
So it's really best to just start from scratch and create your own using the color chart for petz, that way, for the most part, how it looks on the image will be how it looks in the game. They still require more tweaking usually because when they're on balls moving around they look different than your flat image. I had at least 15 reject textures before I got to my final cremello. So it sounds easy but depending on how picky you are it can be really annoying!
I'm using GIMP and it automatically saves it as RGB color when I need it to be indexed color? at least that's what other textures open up as and it crashes my breed Figured it out. Although it looks horrendous lol
This is what happened lol, I guess it's not soo bad, but it's super flat :/ -ignore the horse, it's an WIP mod lol
Also how do I ensure I'm using the petz colors? I keep getting weird reds and lime greens but idk how it keeps happening
I don't know, I have a color chart image downloaded so I open that up in my image editor and then use the color select tool and copy it from the chart and then switch to paintbrush or bucket tool and then they use that color. (I don't use GIMP though, I have Corel PhotoImpact)
Here's the image, courtesy of Osiris, from PCG I think? So I copy 12 from here, and then paint 12 into my picture.
The only time I had strange colors and things happening was when I thought my texture was 128 px and it was actually only 124 px. Then I had strange colored 'blank' areas. So double check your texture size is my only advice to you there.
Textures are such a pain to get to look right, especially the ones for horse coats since they appear on such large balls. I really want to try mane and tail textures but there are already so many that there's no need really haha, but they're probably a lot easier and more forgiving because they take up less space. To get it not flat-looking, you basically just need to make it look like a normal coat texture, like on cali5 when you zoom in to it if the coat is painted 45, the coat ranges anywhere from 42-47 (or something) depending on the in-game shading and highlighting. so it helps to go in and make connected pixels a slightly different colors so that it looks more like in-game fur. It shouldn't be too hard since you'll have so many different shades to choose from like 9 different whites, 9 different blacks/greys etc, my cremello literally only uses 4, maybe 5, different colors since white was the only one with a range of shades, and I couldn't use too much white or it would be too white. So it was a huge pain to get it to look even close to fur.
What might help you more is if you just take a picture of an in-game horse that is the right color and then cut out small pieces of his coat to use to help fill in your texture and make it more fur-like. This won't work for something like roan or cremello where you're creating a new coat color as well as texture, but if you're just trying to do spottiness like on the guy above it should work fairly well. Besides the flatness I think this texture looks really good!
I'm actually not the greatest petz texture expert so if anyone else has something to correct me on or add to?