How do Cephalopods change colour?

A Cephalopod are creatures that can change colours such as Squids, Octopuses and Cuttlefish. Their skin contains cells of the colours red, yellow and brown with reflectors underneath to create the greens and blues. These cells are called Chromatophores and are made to change colour by the contraction of muscles (see image to the right). This action is very fast, and they can change colour in less than a second. These creatures can also change the texture of their skin, to resemble rocks and other bumpy surfaces and they can change their body position to mimic other creatures for protection. For example they can mimic a Lion Fish which is poisonous, meaning a lot of predators learn to stay away from them.  
Today, scientists don't know how exactly they change colour because these creatures are colour blind, how do they know what colour to change into? What is know that it is a conscious action made by the creature to contract its muscles to copy the colour and texture of the environment. These colour changes can also change depending on how the animal is feeling, it can change to warn potential predators or even when they are startled, they turn completely white.
Some Cephalopods can produce light - bioluminescence. However, I've already explored and created the bioluminescence trait in my second creature design so I want to focus the skin colour changes for creature number 3.

Cephalopods have around 3 pattern templates that they use. Above is an image of 3; camouflage for these creatures is about fooling whatever is looking at you, not looking exactly like the background. 

pattern templates:  

a) Uniform - little to no contrast to the pattern 

b) Mottle - Small scale light and dark splodges 

c) Disruptive - to interfere with the recognition of what the animal is. 

Overall, researching into how and why these animals change colour and texture is very interesting to me and I can't wait to explore how my creature will look. Below are links to the websites where I got the images from and did my research. 

http://ocean.si.edu/ocean-news/how-octopuses-and-squids-change-color

http://www.scientificamerican.com/article/how-do-squid-and-octopuse/