I love using patterned paper to inspire the colours I add to an image. Not only does it ensure a cohesive project, but it's also a great way to explore different combinations of colours and shades. To help make the process easier, I always use the colour guide I created for the particular medium I want to use. You can find blank charts on the internet by Googling whatever medium you're using and "blank colouring chart". For example: "Prismacolor pencils blank colouring chart". Try it with your paints, markers and mediums too.