What is the rule about mixing prints or colors together? 

I love prints and have no hesitation in mixing them but I know this isn't in everyone’s comfort zone. A good rule to keep in mind for prints is to stay in the same color family and pay attention to the scale of the print. Typically, mixing a large print with a smaller print in the same color family works well. Again this is something that can be totally subjective and a personal preference too. Solid colors can tie your mixed prints together - for example: large green print in a skirt + small green print in a jacket or cardi with a solid color underneath that can bring it all together. 

As for mixing colors, a good rule is to tie everything together with accessories - for example mixing brown and black together can work well when you add a scarf, necklace or shoes that can tie them together. Remember as always in fashion - there aren't any rules! If you like it wear it. Brands like Gucci and Versace mix prints together as their signature look!