When designing icons or illustrations, you have a choice of colors. Using the right colors is critical to stand out with your icon or illustration set. We recommend checking out the latest design trends in terms of colors and using recommended palettes. A good starter in color theory is this guide: Color Theory: Brief Guide For Designers.
Quick tips for using colors:
Make sure you pick colors that give the icons or illustrations enough contrast.
Always use transparent backgrounds.
When uploading monochrome icons or illustrations, always use the same color for all items in a set.
Use colors that fit the overall theme of the set (for example, Christmas icons could be red).
Examples of icon sets using colors
All glyphs using the same color is the way to go.
Different shades or different colors is not a good idea.
Too many colors used inconsistently will not make a good combination.
Limited color palette and consistent use on the objects inside the set, along with great attention to detail (shading) will add up to great results.
The red color used in the set above can quickly become too aggressive and hurt sales. Additionally, it does not bring value to the set.
All recommendations for icons also apply to illustrations. For illustrations, we generally recommend to make them easy to recolor by customers. This is because customers will almost always want to recolor them to fit their own brand.
Below are some examples of illustrations that are easy to recolor and can adapt to different scenarios:
On the contrary, here are some examples of illustrations that are very pretty, yet cannot be easily recolored:
The colours in the following set are fun and very well chosen, but they cannot be easily edited by customers.
In sum, the choice of colors become less important for illustrations when you know that customers will modify them to their requirements.
Here are a few link that could help you get some inspiration of what colors you should choose for your next set:
https://picular.co/ - This one works like google for websites, but it looks for colors, put in your word and it will look for colors for that word.
https://coolors.co/ - This tool can generate or you can explore a lot of pre-existing palettes.
https://www.huesnap.com/ , https://colorhunt.co/ and http://colormind.io/ are also good color palette tools.