Set background colour of cell to RGB value of data in cell

You can use VBA - something like

Range("A1:A6").Interior.Color = RGB(127,187,199)

Just pass in the cell value.

Setting the Color property alone will guarantee an exact match. Excel 2003 can only handle 56 colors at once. The good news is that you can assign any rgb value at all to those 56 slots (which are called ColorIndexs). When you set a cell's color using the Color property this causes Excel to use the nearest "ColorIndex". Example: Setting a cell to RGB 10,20,50 (or 3281930) will actually cause it to be set to color index 56 which is 51,51,51 (or 3355443).

If you want to be assured you got an exact match, you need to change a ColorIndex to the RGB value you want and then change the Cell's ColorIndex to said value. However you should be aware that by changing the value of a color index you change the color of all cells already using that color within the workbook. To give an example, Red is ColorIndex 3. So any cell you made Red you actually made ColorIndex 3. And if you redefine ColorIndex 3 to be say, purple, then your cell will indeed be made purple, but all other red cells in the workbook will also be changed to purple.

There are several strategies to deal with this. One way is to choose an index not yet in use, or just one that you think will not be likely to be used. Another way is to change the RGB value of the nearest ColorIndex so your change will be subtle. The code I have posted below takes this approach. Taking advantage of the knowledge that the nearest ColorIndex is assigned, it assigns the RGB value directly to the cell (thereby yielding the nearest color) and then assigns the RGB value to that index.

Sub Example()
    Dim lngColor As Long
    lngColor = RGB(10, 20, 50)
    With Range("A1").Interior
        .Color = lngColor
        ActiveWorkbook.Colors(.ColorIndex) = lngColor
    End With
End Sub