Learn Spanish (Spain): Las casas viejas son blancas.

They are describing the noun casas (houses). Because casas is feminine and plural, every adjective describing it must also be feminine and plural. The base forms are viejo and blanco, and changing the "-o" to "-as" makes them match.

Why is the order casas viejas instead of viejas casas?

In English, we put the adjective before the noun ("old houses"). In Spanish, descriptive adjectives like viejas usually come right after the noun they describe.

Does blancas still have to match casas even though it comes after the verb?

Yes! Even when an adjective is separated from the noun by a verb like son (are), it still has to agree in gender and number. It is describing the feminine plural noun casas, so it must be blancas.

Why use son here instead of están?

Son is from the verb ser, which we use for permanent traits and physical descriptions. The color of a house (blancas) is considered a basic characteristic, so we use ser rather than estar.