This is an example of good contrast used in a poster. The contrast between the tone and color of the images in the background and the text indicate which takes priority. If they were the same tone and and hue, it would be hard to distinguish the difference. Also the contrast between the size of the text gives variety and brings direction to the text (flows downward). Also, the background is very creative. Poster looks like its printed on a notebook with very raw hand sketches. Also the lighter background and the darker text create a sort of contrast in depth.
This is an example of a bad poster design that has no contrast to bring out the important information. There is no contrast in depth. The way the white text is just place all over with no given space closes everything together making it look rather flat. There is no contrast in color or tone. The white text kind of flushes together and the tone doesn't bring any depth. There is a little contrast in size when it comes to the text but aside from the huge '30' all the text kind of merges together. Also using so many different type fonts detracts from the poster and it is rather hard to read.
[IMAGE CREDIT: Graphic Design && Big 30]