The colours are based on what is visible on the screen. The area with the darkest shade of blue represents the lowest average and red represents the highest average. If you move to a view to include very expensive properties, expect most of the colours to shift towards blue because there is a new high value.
Some postcodes are omitted because there have no been enough sales to provide a reliable average. In these cases, the nearest neighbour will fill in the gap.
The house price data you see is from the Land Registry and only covers England and Wales. The property registrars for Scotland and Northern Ireland do not make their data freely available.
To make the site load as quick as possible, we estimate the boundaries between postcodes by their proximity to one another. Where there isn't another postcode in the way, the area covered extends to the edge of the screen. By the coast, this is usually out to sea.