Another option besides the ones suggested: recolor it manually. This can be also be used to "tweak" colors if a plugin doesn't make it come out correctly.
Step 1: turn your image to black and white. Save a copy, then export a "flattened" version to work with. If tweaking colors, use your colored image as a base instead!
Step 2: Put your image on the bottom layer of a brand new image.
Step 3: make a new transparent layer for each main things you want to color. For example, "tree leaves" and "tree bark" should be two different layers to make life easier on you.
Step 4: Color the section's layer like a coloring book. Use the pencil tool with anti-aliasing OFF. This makes it so you can use the bucket tool later if you find out you need a different color than the one you picked. Save as you go, just in case.
Step 5: Set the colored layer to "multiply", "hard light", or "soft light" (play with these until you find the right one).
Step 6: Play with the opacity of the colored layer until it looks right.
Repeat until it's all colored!
Use a brand new clean scan in a lossless format (ex: .PNG) every single time for best results. Yes, files can have errors in them, especially if they were in a lossy format like .JPG or .WEBP (which can be the worst IMHO). Stuff saved off web sites often are in lossy formats in order to save server space. A lossy format is small, but mistakes happen in the files over time. This makes the images less clean and harder to make nice.
An AI or plugin is basically doing the recolor tactic from above, looking at the data in your picture to "guess" where to put the colors. You can try to use the AI colored ones along with your hand made work to try to make life easier. Sometimes, that is useful. If you have access to a tablet (even if it's small and cheap), use it: it will help you be more accurate.
How to turn an image black and white:
I simply used "desaturate" to get a black and white version. Feel free to try "desaturate color to grey" as well, as sometimes one works better than the other. From there, I made four layers all of the same desaturated image but using different modes. Each layer does something different to help the overall look. If this isn't quite your taste, try making your own using "desaturate", then make copies of the layers and play with them until the image looks nice.
My layers:
Top layer: exclusion, 30% opacity.
Layer 2: hard light, 56% opacity.
Layer 3: lighten only, 50% opacity.
Bottom: normal, 100% opacity.
Your bottom layer always needs 100% opacity as a "canvas" for everything to build on. When you use multiple layers in different modes, you can make small changes that build up like paint on a canvas. Play with different layering and modes to find your best version.
![[Image: 1-desat.png]](https://i.postimg.cc/rm9wWq28/1-desat.png)
