Small colofull post its inside Cards flagged visually outside on board
Add small annotations, like Post Its smaller and with other color inside Yellow cards,
Just like a visual quick reminder of small actions and or ideias on the Card
Thanks Bruno for the suggestion!
There's also an idea for colored labels on cards here: http://storiesonboard.uservoice.com/forums/276721-general/suggestions/6950016-custom-colored-labels-on-cards, that sounds similar to me.
What do you think, is it the same? Can I merge the two?