Let me explain why I opted for the "industrial fertilizer" color: the first time I tried Fertile Fields out it took me a while to get the difference between the shades of green used in the compost vs. the fertilizer. See, in real life you'd apply compost directly to your crops. So, without seeing them both side by side, at first I didn't realize they were two separate things! And that in an already complex new system, with the terraforming and all... Making this difference more easily discernible visually was one of my goals with this mod. Since sand, dirt, compost, fertilizer and ash are all powders, it made aesthetic sense that they shared the same form. Differentiation had to come through color, so that means one distinct color per resource. Between green compost and brown dirt (which is missing in the preview image), there was not much space to fit the fertilizer original hue in. Going light brown would conflict with the sand, and this stuff's real dark color also could get confused with the dark grey ash. That's why you end up with the fertilizer color you'll find on a google search! At least its relatable to our world.
But...
If you're willing, it's actualy pretty easy for you guys to change it for yourselves! I'll tell you how to do it:

1. Locate the mod's patch folder on your hard drive. Typically that's C:\Program Files (x86)\Steam\steamapps\workshop\content\294100\1653937715\Patches
2. Open FertileFieldsMakeover.xml in your Notepad, then find <texPath>Fertilizer</texPath> and replace it with <texPath>Ash</texPath>. Don't touch anything else, save and exit.
3. On that same folder, create a new text file and name it JustForMe.xml (NOT .txt, don't let windows fool you, turn on "File name extensions" to be sure). On that file, place this code and nothing else:

<?xml version="1.0" encoding="utf-8" ?>
<Patch>
<Operation Class="PatchOperationInsert">
<xpath>/Defs/ThingDef[defName = "Fertilizer"]/graphicData/texPath</xpath>
<value>
<color>(67,100,50)</color>
</value>
</Operation>
</Patch>

Those numbers make the RGB color Rainbeau used on the original mod, you can fiddle with it if you want to.
4. Restart the game and enjoy!

And welcome to the most easily moddable game there is!