Add the following:

$Sheet->Cells(22,31)->{Interior}->{ColorIndex} = 6;
[download]

Update: it should also be possible to do:

$Sheet->Cells(22,31)->{Interior}->{Color} = $Excel->RGB(255,255,0);
[download]

Update 2: you can pack the RGB values manually and this seems to work:

$Sheet->Cells(1,1)->{Interior}->{Color} = ($blue << 16)
                                        + ($green << 8)
                                        + $red;
[download]

Hope this helps!