Tell your friend you are not that interested in old paintings. Actually you rather like modern paintings.
Try to use 其实 and 倒。
我对古画并没有兴趣，其实我倒对现代画很有兴趣。
[Comments]
Click on the Chinese to hear the sentences spoken.
Alternatively one can say 我对古画没什么兴趣,我倒对现代画比较有兴趣。