Ive never used Ibis Paint X, but if its anyhing like the editors I have used, this is my general process.
*Disclaimer : Ive found that no two color changes are exactly alike, and therefore you my need to play with all of these settings until you have something you like. This may take a lot of trial and error. Ive found some pieces that just dont cooperate and never look quite right.
I find bay or chestnut work best to turn into a black. (If I cant find a black I like.) The method I use is the same for either, except that if you are using a bay, you will need to exclude the black points, and make sure that as you edit the brown parts to black they lend into the black in a naturalized looking way.
To begin with, I usually lower the saturation most of the way. I will add hues back into it later, so for now its fine to zap the color right out of it.
I then use brightness/contrast, and hue/saturation/lightness to darken the coat. I usually up the contest a little, while I take brightness and lightness down.
*some areas of the coat may need more darkening than others, depending on the shades of the original color*
Next I usually then use color adjustments to add a bit of a blue hue to it. You may prefer to use a different color(s) depending on the piece. You just want to add in some of the color that we have previously taken away.
My last step it to add the horses details back in. Color changes almost always take away details, no matter what the change is. On new layers, paint back in the shadows and highlights, using off blacks and off whites.
As far as art apps go, I donÂ’t know if you have a computer available, but if so I would suggest gimp if you are looking for something free, or Photoshop if you are willing to pay. Photoshop CC is about $10 a month.