Clouds in colored pencils are like clouds in any medium. Really pay attention to values and shapes, then build up mixes of colors to get the subtle shifting neutral hues in them. The color of clouds varies a lot with the terrain and the color of the light, the time of day, latitude, everything will affect it.
Working slowly from a good photo reference helps. Some clouds have very intricate shapes. It's surprising how hard it is to get good cumulus clouds, the puffy ones are among the hardest to do. If you get Jack Hamm's book Drawing Scenery: Landscapes and Seascapes,
which is about $10 new on Amazon, it has a section with drawing every specific type of cloud known to meteorology.
The ones I get best are the mare's tails -- the drifty little cirrus clouds that attenuate sideways. I draw the spaces between them in my sky tones -- first I use a mid blue like Prismacolor True Blue shaded solid at the top and fading to barely there at the horizon, then work through successively lighter blues burnishing what's above it and fading out as I get to the horizon, then get to Cloud Blue at the horizon.
Using horizontal strokes, I feather the ends of each stroke. I touch down lightly moving in the direction the clouds will be going to do a space between them. Then I press hard in the middle and keep going lifting the pencil gradually. Using a blunt point helps with this feathering effect. I go back and forth, over and over, varying the lengths of the strokes so the cloud shapes vary but creating them by drawing what's between them -- and shading into them.
This leaves some white space that is perfect white little mare's tails. If I want the painting burnished, I'll go into those with white and burnish them out toward the blue parts of the sky. Then maybe touch in a little very light yellow as a glaze over the white, depending on how golden the sunlight is. Cirrus clouds don't usually have dark shadowed undersides like cumulus.
Here's a quick example done in my sketchbook with Progresso Woodless colored pencils.
Clouds Quick Cirrus
Koh-I-Noor Progresso woodless colored pencils
Wirebound ProArt sketchbook paper, about 4" x 5"
I didn't put another bunch of them close to the horizon, but any clouds will be much smaller near the horizon than at the top of the page. That's perspective and it really helps make clouds look more real. This kind of cloud works for most landscapes and is lots easier than cumulus, at least for me. I like having a good photo reference to try for cumulus, or working from life if I'm outside and can see some interesting clouds.
Try sketching them with just a dark color like black or blue or brown, get the shapes right by way of values and then overlay different colors of the right values for a full burnished colored pencils painting look. This is just a sketch in color showing shading between two blues -- the set didn't have a super light blue so I would burnish the bottom with white if I were to polish it down.
I added a little yellow to the leading edges to the left and a little pink over the middles so they're dawn clouds, because that's prettier than just white. Look at the time of day and exact hues in your reference or out in life. It's good to do a lot of quick cloud studies outdoors till you get used to the shapes and colors for different times of day in your location.