Illustrations are used to visualise the topic in question, for an easier overview and explanation of a complex matter.
Similar to icons, the illustrations continue using the line style. The detail level can vary depending on the complexity of the topic. To balance out the delicate lines, a contrasting solid shape of colour gives the illustration more weight and visibility.
Combine a 3px thick line with a solid colourful shape. Use the colour shape in only few elements of the illustration, keeping most of the details in line style.
It is recommended to use colours from the extended colour palette. However, in certain occasions, it is allowed to use any colour that fits the purpose better.