Tombow brush pens come in 96 colours and different sets including Primaries, Secondaries, Pastels and more. They are twin-ended, a brush nib at one side and a bullet at the other. They're odourless, acid-free and don't bleed. This, combined with their blending capabilities, make them a great alternative to alcohol markers which won't saturate the paper and ruin any designs on the other side.
One neat little feature is that the lids will both fix to the opposite end for safekeeping when you're colouring, so no more lost lids or having them roll off the desk!
I love that the blending doesn't affect the lighter pen at all- their self-cleaning tips mean you simply scribble out what's left of the darker colour and the ink will appear as fresh and bright as it was when new. Colours are true to the cap and can be layered for deeper colour with minimal feathering of the paper.