I know this suggestion might seem a bit weird but I've discovered you can totally use old/discarded/not-really-liked eyeshadows to decorate journal/planner pages. They make pretty and colourful smudges, you can use them to decorate edges of the page, space around stickers or below/above washi tape. They don't damage pages in any way, all the writing, stamps, doodles are still perfectly visible.
And the best thing? Even cheap eyeshadows work! In fact, I've discovered the cheap, no-name ones seem to look the best (maybe because I'm not worried about "wasting" them so I'm more likely to experiment with them than with the better/more expensive ones). Plus, you can have them in so many colours! They can competely change the way your journal/planner looks, changing a bland ("Give me a break, I was so tired on that day") page into a pretty one :)
One warning though: decorate the page AFTER you finish writing; otherwise your pen may not work on the pigmented paper.