You have models, and you have a list in mind.
How do you make these models work together? Often, the models from the same army are similar in nature. Imperial Marines of all flavors are a fine example of this. But what happens when you're playing imperial guard and you're tired of painting just Cadians? Or you play daemons? The Gods of Chaos favor certain color schemes, and you are left with a plan to make what seems like an explosion in a paint factory.
What do you do? The key is a proper selection of colors.
My guard army contains many elements. I have Tallarn, Valhallans, Cadians and even a Mordian sergeant thrown in for good measure. With a casual glance, you'd never notice it, due to the army's unified color scheme. Let me show you:
Even though the models have different colors, poses, and body styles, they all reference the main color: olive drab. It allows for differences amongst the models to blend into a cohesive whole. The same goes for any other guard army, and any other color choice. Imagine an army of Tallarn in white tunics with black turbans. Pair them with Sisters of Battle from the Order of the Sacred Rose. Though the armies are different, their cohesive color scheme will make them a stunning army to field or face.
But what about the daemon army? Slaanesh loves his/her/its pinks and purples, Khorne, his reds, etcetera. How do you blend them? We turn to fashion advice for this. For maximum impact, one mixes vivid colors and neutrals together. The colors should be an accent to the larger neutral palette. What is a neutral color? A neutral color is one not found on the color wheel: black, white, grays, beige and other earth-tones.
How does that knowledge help us bring those disparate elements together?
Model by Ferix on DakkaDakka:
The skin on this daemonette is a neutral shade, leaning toward a white/beige, with the blue accents becoming the accent.
You could easily pair that deamonette with the following bloodletter, if one tweaked the colors.
Model by Catpeeler on DakkaDakka:
Another example of a neutral fleshtone comes from my own daemon army, the Host of Hope's Demise. Though it would not fit with the above two examples, I would have no trouble painting a daemonette's flesh in the same earthy flesh tones.
The neutral flesh you choose for your army will help it blend, while each squad will show the colors of it's patrons, giving both players a quick visual shorthand to each unit's nature.