Colour adds a great visual punch to your poster. As powerful as colour is, it must be used carefully.
Aim for high contrasting colours. Avoid black text on an all white background as that can cause glare and eye strain.
Too much colour, as seen in the poster below, and the content is lost. This poster is too hard to read and too visually cluttered – from the busy background, the coloured boxes, the Star Wars like title… just too much.
The poster below that, has just the right colour elements. The text stands out from the background and the coloured text boxes. The application of colour is consistent as well… font, background, textboxes, etc…