Below I set the font color to red and the background of the cell to green.

Code:
IDispatch *pXlRange1;
{  
VARIANT range;
range.vt = VT_BSTR;
range.bstrVal = ::SysAllocString(L"C1:C3");
VARIANT result;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYGET, &result, pXlSheet, L"Range", 1, range);

pXlRange1 = result.pdispVal;
}


IDispatch *pXlInside;
{  

VARIANT result;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYGET, &result, pXlRange1, L"Interior", 0);

pXlInside = result.pdispVal;
}
{  
VARIANT color;
color.vt = VT_I4;
color.lVal=RGB(0,255,0);
VARIANT result;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYPUT, &result, pXlInside, L"Color",1,color);

}



IDispatch *pXlFont;
{
VARIANT result;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYGET, &result, pXlRange1, L"Font", 0);
pXlFont = result.pdispVal;
}


{  
VARIANT color;
color.vt = VT_I4;
color.lVal=RGB(255,0,0);
VARIANT result;
VariantInit(&result);
AutoWrap(DISPATCH_PROPERTYPUT, &result, pXlFont, L"Color",1,color);

}