First of all, you need to understand the basics of editing an image.
Secondly, get any image editor software such as Photoshop or GIMP, add in the notes you want to edit.
For GIMP, right click the layer of the notes and find "alpha to selection".
With that, you can add a new blank layer and freely choose any colour you want. Export it as png, keep the filename the same and replace the original image.