Barrica/Barrika is a small municipality about 10 km from Bilbao in Biscay province, northern Spain. According to this unofficial Barrica website (Spanish and Basque text only), the meaning of the colours is basically:
Green: the land (Basque barrika means "field," "small forest" or "newly established settlement on an open slope")
Dark blue: the sea
Light blue: the sky
Light and dark blue are displayed on two horizontal stripes to represent the landscape which can be seen most frequently from the town. Since the hoist is the point whereby the flag is joined to the land, the latter is symbolized at that point with a green hoist triangle.