One of the most flexible materials you can use for 3D printing is thermoplastic polyurethane (TPU). When it comes to mixing materials and colors, it will mostly come down to trial and error (and lots of practice!). My top tip would be, if you’re using more than one, to try and stick to materials that reach similar temperatures during printing. Avoid, for example, using PLA and ABS as these are more likely to warp or result in adhesive issues.