red, tan
冉 centers on a reddish-brown color: red or tan. The meanings are closely related as shades of red.
冉 is an ancient pictograph whose exact origin is uncertain. It may have depicted something hanging or drooping, but its connection to the color meanings is unclear.
The shape 冉 looks like a piece of red cloth hanging from a rod, with the vertical lines as folds. Picture a tan banner fluttering in the wind.
For ゼン, imagine a Zen monk wearing a tan robe: Zen -> ゼン, and the robe is the color of 冉.