Personally, I would use polymer clay, thinned with liquid clay to a mushy consistency. You get absolute compatibility and color matching this way. Use thin sheets of marbled clay for large areas, too. You can do a lot with ropes and blobs of solid clay, etc...
Hi Marica, I use TSL (Translucent Liquid Sculpey) mixed with small amounts of Genesis heat set oil paints and sometimes with a little mica powder like Pear Ex mixed in. (if using Kato Liquid Poly clay gently stir VERY WELL first) I keep the mixtures in sets of small metal screw top jars they sell for organizing little scrapbook treasures. For info see #12 and accompanying two photos at http://melobeau.blogspot.com/2012/10/part-2-my-25-favorite-polymer-clay.html
Genesis paints heat set when the PC is being cured and it's commonly used with PC. Though the individual jars of Genesis are relatively expensive, there are sampler packs of small amounts in plastic tubes available (though there could be some important colors that might be missing).
I have used Reeves oil paint mixed into Translucent Liquid Sculpey. Then I bake the final project (this will never dry on its own). The more oil paint you add to the liquid polymer the thicker the consistency will be (this has a basic consistency of craft acrylic paint). I would say that the color lightens only a little. I have not used this strictly as a painting medium. I use it for detail on miniatures. I usually mix up batches of the color and keep in small containers so I know I have plenty on hand. I have found this to be very cost effective since a little goes a long way for my little creations. Here is a link that talks about using oil paints with polymer clay: http://www.polymerclayweb.com/Materials/InksPaints/OilPaints.aspx   Now, I wouldn't use oil paint straight on the clay 98% of the time. It will take much too long to dry (if ever). I have always fantasied about getting a full set of oil paints and the same number of large TLS bottles and just emptying the tube of oil paint in each. Then I would be swimming in the stuff. Using the squeeze bottles to hold the tinted liquid polymer is also a nice way to dispense the "paint" with less mess and waist. If you want something that has more of the thick paint feel that you can scoop on with a paint knife, then mixing solid polymer clay into the liquid polymer clay will give you that feel (this will have to also be baked to set).
I paint all of my sculptures with Americana acrylic and/or Dick Blick and Luquitex tube acrylic paints. I use white sculpey, and have even painted the colored clays. I prepare the clay for painting with gesso to give the clay 'tooth'. I seal my sculptures with Mod Podge.