Post History

I failed to notice that a cell's `CellStyle` property returns the generic `ICellStyle`, which has to be cast down to `NPOI.XSSF.UserModel.XSSFCellStyle` in order to gain access to the `FillBackgroundXSSFColor` and `FillForegroundXSSFColor` properties.

Also, here's a summary of which property does what, quoting from [this answer]():

> As said in my linked answer: Cell interior uses pattern fills. The fill background color is the color **behind** the pattern.The fill **foreground** color is the color of the pattern. To fill the cell using a plain color, you need using fill **foreground** color `CellStyle.setFillForegroundColor` and solid pattern `FillPatternType.SOLID_FOREGROUND`.

In my case, I need `NPOI.SS.UserModel.FillPattern` and `FillForegroundXSSFColor` then. The full code:

```
let setCellColor (cell: ICell) (rgb: byte array) =
    let xssfCell = cell :?> NPOI.XSSF.UserModel.XSSFCell
    let newCellStyle: NPOI.SS.UserModel.ICellStyle = xssfCell.Sheet.Workbook.CreateCellStyle()
    match xssfCell.CellStyle with
    | :? NPOI.XSSF.UserModel.XSSFCellStyle as xssfCellStyle ->
        let color = new NPOI.XSSF.UserModel.XSSFColor(rgb)
        xssfCellStyle.FillForegroundXSSFColor <- color
        xssfCellStyle.FillPattern <- NPOI.SS.UserModel.FillPattern.SolidForeground
    | _ -> failwith "'newCellStyle' cannot be cast to XSSFCellStyle"
    xssfCell.CellStyle <- newCellStyle
```

To call with RGB colors:

```
let getCell (workbook: NPOI.XSSF.UserModel.XSSFWorkbook) (sheetNumber: int) (address: string) =
    let sheet: ISheet = workbook.GetSheetAt(sheetNumber)
    let cellAddress = new CellAddress (address)
    getCellByAddress sheet cellAddress

let rgb: byte array = [|255uy; 192uy; 150uy|]

setCellColor (getCell o 3 "a7") rgb;;
```

To call with hex color string:

```
let hexStringToRGB (hexString: string) =
    let isHexColor (color: string) =
        "^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$"
        |> fun regexPattern -> new System.Text.RegularExpressions.Regex(regexPattern)
        |> fun regex -> regex.IsMatch(color)
    match (isHexColor hexString) with
    | false ->
        failwith "Invalid hex color string."
    | true ->
        let hexString = hexString.TrimStart('#')
        let r = System.Convert.ToByte(hexString.Substring(0, 2), 16)
        let g = System.Convert.ToByte(hexString.Substring(2, 2), 16)
        let b = System.Convert.ToByte(hexString.Substring(4, 2), 16)
        [|r; g; b|]

setCellColor (getCell o 3 "a7") (hexStringToRGB "#ffc096");;
```

> NOTE: `setCellColor` replaces the input cell's style with a new one, so one has to take care of re-creating the previous style options if they are needed.
> When I modified `setCellColor` to modify the cell's own style object instead of replacing it, the result was that the entire worksheet got painted to the color used from that specific cell going forward. Not sure where I screwed up, but beware.
>
> **update** See the solution in [How to change the color of a cell to a user defined value while keeping its existing style using NPOI from F#?](https://stackoverflow.com/questions/77705384/how-to-change-the-color-of-a-cell-to-a-user-defined-value-while-keeping-its-exis).