Although I still draw with pencils and pens and brushes, when it comes to final versions of artwork going to print, I have to go to the computer, because today it is more reliable when it comes to the integrity of colour. Last Century we used inks, watercolours and an array of emulsion techniques to create artwork that had to be scanned and separated into plates for the printing press. Today, our files can generate process colour separations automatically. It's built in! The only trouble is the display (monitor) may not be showing the colour the way it will come off the press. That's where the notion of "reliability" comes in.
After years spent in the traditional printing business, I see colours as percentages of CMYK. So when it comes to my illustrations, I still use colour mixes based on press proofs I saw 20 and 30 years ago. That's because even though the technology of the press has changed in terms of plates and inks, the notion of getting colour on paper is still the same. No one has legitimized alchemy or time travel for that matter . . . yet. Colour mixes have not changed. It's not as if you can put yellow and cyan together and get red. That's like putting gin and tonic together and expecting rum and coke.
Beginning an illustration starts with a few big shapeless blobs in a vector-based, page description language such as Postscript (that would be an application program such as Adobe Illustrator). A green, subtle gradient becomes the grass. A pale cyan gradient will be the sky. Keep them on separate layers so you can turn them on and off when necessary. Believe me. It will become necessary (some of my drawings can go into hundreds of hours). When the topographical detail of a landscape starts taking shape, I adjust the gradients for contrast.
I try to keep it looking clean and simple, but some of the small stuff is labour intensive. Tiny details are where the fun is. Even though they are not always caught by the glancing eye, their influence is still processed by the graphic part of the mind. The result is it still becomes broad strokes vs. small strokes. All of a sudden, we're back in the art world again with old school, traditional painting. The brush may have changed and the colour has gotten less toxic and doesn't leave a mark on the floor anymore, but light, shadow and midtone are all still there - just as reliable as the sun and the moon.