how to set background color of an excel cell using java script

No, that wont work, since .ColorIndex is expecting a number, rather than text.

If I simply record a macro of me setting the bkg colour of a cell, I get the following code:

Sub colorBkg()
'
' colorBkg Macro
'
'
    With Selection.Interior
        .Pattern = xlSolid
        .PatternColorIndex = xlAutomatic
        .Color = 192
        .TintAndShade = 0
        .PatternTintAndShade = 0
    End With
End Sub

This is for the darker of the 2 reds available in 'standard-colors' of Excel2007 (rgb(192,0,0))

The method for computing the number used for Interior.Color is:

Red + (256*green) + (65536*green)

With that in mind, consider the following modification to the recorded macro which sets the colour of any selected cells to rgb(100,100,100)

Sub colorBkg()
'
' colorBkg Macro
'
'
    Dim r, g, b
    r = 100
    g = 100
    b = 100
    With Selection.Interior
        .Pattern = xlSolid
        .PatternColorIndex = xlAutomatic
        .Color = ((b * 65536) + (g * 256) + r)
        .TintAndShade = 0
        .PatternTintAndShade = 0
    End With
End Sub

So, I would expect that you'd need to change your code a little. If I use the object browser, I get the following values for the constants:

Const xlAutomatic = -4105 (&HFFFFEFF7)
Const xlSolid = 1

You don't appear to need to set the last 2 attributes, so you could probably get away with:

excelSheet.Cells(i + 1, 1).Interior.pattern = 1;
excelSheet.Cells(i + 1, 1).Interior.PatternColorIndex = -4105;
excelSheet.Cells(i + 1, 1).Interior.color = 255;

Note: I cant remember if the property names are Case-Sensitive from javascript or not.