There is a scene in the movie, "The Girl With The Pearl Earring" where the Dutch painter, Vermeer opened the shutters and asked his young model to look at the clouds and tell him what color they were. She looked and replied, "White." He gently said, "Look again." and the look of wonder on her face was amazing as she listed the many colors she discovered after really looking.
If you're painting plein air (outside, essentially) and you're actually looking at clouds, you'll notice there are different kinds. If you see the beautiful, large puffy marshmallow variety, you're seeing cumulus nimbus' clouds. And, you better paint quickly as they often roll in before a storm. Maxfield Parrish painted some of the most amazing cumulus clouds, check his work out on the net. While you're at it, find a book or site that shows the many kinds and shapes of clouds. Unless you're trying to make a statement, you should know that cumulus clouds are rarely seen in arid areas, for example. Notice how clouds sometimes look very flat on the bottom, or even much darker, or the different shapes and layers. I guess I can be kind of a menace when driving, because I do become distracted by awesome cloud formation. One day, as I was driving home from the grocery store I notice a couple of cars on our two lane country road pull over and point at the sky. There for only about half a minute, was what looked for all the world like a giant angel, wings and all, drifting over the farmers fields. That was a great experience because I knew other people appreciated (or were freaked out) by it too. Observation and taking notes is very important, but be careful and watch where you're going.
Consider your medium (not that blonde lady on TV). Painting clouds with oils or acrylics will be much different than painting them in watercolor. With oils or acrylics the background is painted first and the clouds, since they are usually lighter than the sky, are painted on top. Look at various cloud formations and choose your favorites. They're fairly easy to paint once you really see' the shapes and the many colors they contain. I have a photo of rose and peach colored clouds taken at sunset that people insist that I've touched up' with on the computer, but I didn't. There are red, purple, orange,ivory, yellow and even green clouds. However, don't linger and sky-watch if you see green clouds as they are usually found hanging out with severe storms and tornadoes.
With watercolor, I usually under-paint the sky with a pale yellow or very, light or no color at all. I paint a wash of my sky color (which can be just about any color when you think of it) and lift the clouds out with a tissue or a sponge and then add a few details with light layers of wash and dab a little of each color off in places.
Make sure you know where the sun is in your painting. Clouds seen at sunrise, mid-day and sunset will look very different. Watching sunsets is one of my favorite things in the world. And every part of the planet has their own special clouds'. And you can only paint your interpretation of the clouds you see. Have fun!