Bleeding is a term given to the occurrence of leaking or merging of liquid colour when used on some surfaces. Bleeding can be accidental, particularly when using colouring pens and graphic markers on paper that is unsuitable or non-bleedproof, which can decrease the quality of an image. However, bleeding can also be intentional when ‘bleeding’ or merging colours together to produce some interesting and attractive effects.
Ink and liquid paint will naturally follow the fibres in the paper, depending on the type, to create a varying, jagged edge as colour expands around the original mark made. If this is not what is intended, look at choosing the best surface option for the colour and technique you are using.