Subject: what has the properties of colour change with oxidation?

i am interested in investigating substances which could be used to dye paper 
which one could from that moment on, tell easily from colour the length of 
time elapsed since dying. time period of interest one month; and i assume 
oxidation would be an easy and accessible (and safe!) way to acheive this 
effect. This is an important part of a wider project but as you can tell 
chemistry is not my usual area of knowledge. much thanks.

Re: what has the properties of colour change with oxidation?