Tickets are assigned a colour depending on the order they are bought. For example, the 1st 300 will be colour pink, next 300 will be green and the last 400, yellow. These are just examples and may not be the colours we end up using! Anyone with a ticket in a particular colour can sit anywhere they like within the seats in that block. That way if on day 1 you choose a seat you don't particularly like you can alter it for day 2.
Hope this helps explain it a bit more!