Oil paintings are the hardest to photograph. There are chemicals and fluorescers in the paints themselves that reflect light differently. The colors recorded on film may be so far off from what your eyes see. Remember film records light that our eyes cannot see. As a wedding photographer for more years than i care to remember ... How many times have had to explain why the brides white dress is slightly purple or pink or bluish when the color balance is perfect. I have even dodged a print with a CC filter trying to eliminate this. Modern oil color paints have many color formulas that fool your eyes ... that cant be seen or are different in color from viewing on film verses original. LOL Dont forget the UV CUT off filter and polarized light source. Photographing oil paintings are a very specialized part of the photo biz.
this is a quick answer http://www.wikihow.com/Photograph-Oil-Paintings