How to cusotmize the grouped list title backgound color

valderes-squadra 

Yeah, I completely failed to achieve your objective. Let me try again.

I was going to give you a bunch of reference links, but it looks like you have found the ROW FORMATTING / LIST VIEW CUSTOMIZATION help and have gone through it.

So I have JSON below that should help, I hope with your group header row formatting. Let's try it.

Note that the "$schema" property value for COLUMN FORMATTING is different from ROW FORMATTING. This view customization is for row formatting

{
   "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/row-formatting.schema.json",
   "groupProps": {
      "headerFormatter": {
         "elmType": "div",
         "txtContent": "= @group.columnDisplayName +': ' + @group.fieldData.displayValue",
         "style": {
            "background-color": "=if(startsWith(@group.fieldData, 'abc'), '#e0ffe0', if(startsWith(@group.fieldData, 'cde'), '#ffe0e0', ''))",
            "font-weight": "bold"
         }
       }
    }
}

I developed this from reading what you did. Things to be careful about

• if you don't have 'txtContent' property set, you won't see any text. What happens above is that the column name is put, then a colon, then whatever is displayed as the value in the column that was used in the grouping (the column when you created the view)
• You want to background color set on a dependency. I used basically an Excel formula style with the allowed functions. In this case, I used the 'startsWith' string function. If you are looking for containing substring (not starting with, try 'indexOf()' function, and look for values that are not negative (-1).  Your particular substring is 2nd argument in 'startsWith()'. Add any other CSS styles that you want. I had to use "font-weight":"bold" to get the boldface in the group header text
• For multiple conditions for a background color, you have to use nested if() tests. Keep track of the if() three parameters [test, true-condition, false-condition], and all the parentheses, as well as alwys using single quotes for any literal strings. Also JSON does not allow line breaks in strings, so if you have a large set if if() tests nested, join them all into one line for that string 

Row and column formatting and general list customization require a lot of trial and error to figure it out. Make sure you are using the advanced mode setup with Preview/Save/Cancel editing for very quick results/testing.

If you dont see what I am seeing, reply back.