These posters both advertise documentaries being shown on Channel 4. There are several conventions that all Channel 4 posters share. First, the use of a large memorable image or several smaller images that hold significance and obviously tell the viewer what the documentary is about. The famous Channel 4 logo is placed somewhere on the image and depending on the colours of the image, it can be different shades and colours. It is important for the colour of the image and the colour of the logo to completely clash making the logo as eye-drawing as possible to make sure the viewer identifies the documentary as a channel 4 show. The poster usually includes a catchy slogan that relates to the topic of the documentary. This is written in Channel 4's own font called C4 text. It is usually a very short sentence, usually written across two lines, then placed in a text box the opposite colour to the colour of the text. The same rules and conventions on colour from the Channel 4 logo apply to this text. The title, the date and the time it starts are then written somewhere in the exact same layout but slightly smaller. The slogan is usually placed close to the top with the title placed lower down in the poster. Finally, the poster may often include the logo of a sponsor or affiliated company.
Style guides for Channel 4 advertisements can be found here - http://www.channel4.com/about_c4/styleguide/
