Colour maps are generally more expensive to print and, unlike covers which are often prepared in advance of the supply of the manuscript, are usually commissioned when the script is already going through the editorial process and at various proof stages. Some maps have been commissioned a few weeks before the book goes to press and the worst case scenario is when there are only days. Colour maps can take longer to make depending on the complexity of the colour scheme. For these reasons colour maps are rarely requested. Sometimes they might be made after the book is printed for promotional material or as a memento. Colour can be added to any black and white/greyscale map. The colours are laid over the background in editable layers. Those colours can also be separated for print using pantone colours for instance, or a combination of inks (CMYK). You can also use a single ink, for instance converting greyscale to a sepia effect, which is cheaper to print but still can be very effective.
All my maps are digital with labels and other parts on separate layers. However, I like incorporating hand drawn features. I believe this makes the map feel warmer and more personal to the reader. In some cases I deliberately make the map more naïve and obviously hand drawn because if you make the map too neat and tidy it can come across like a cold engraving or stylised in a way that might remind you of a certain type of comic. A map can become more realistic and sometimes grittier if it is untidy, just as in the same way real handwriting is flawed as opposed to regular and standardised computer fonts. I don’t use any particular style though. I’ve used old fashioned calligraphic pens or modern drawing pens, brush and inks... If someone has an idea for something new I’ll consider giving it a go. I like variety, I like things to look different. To me a lot of the computer generated vector maps even though they may be done by different illustrators can tend to look the same. In the end style is a matter of personal taste and it has to be noted I do not make the maps for myself they are commissioned for a particular book (with limitations on cost, time, etc).
Vector line maps can be simpler to make, easier to correct, etc. They can also convey the idea of the modern world and you can stick more information on them such as labels, or contours, etc. On the whole I think maps created purely by computer are colder, but they are practical and functional. They are quite often used for historical novels, even though they don’t have a period feel, because they suggest the details featured on them are historical facts, impersonal and timeless. So you can have battle plans with triangles, squares and sans serif fonts and it helps to get details or information over without the reader being distracted, wondering whether the border or the lettering are the right shape/authentic for that period.