The colours used in each word list are based on PLD's stages used in our Scope and Sequence book.
The alternating colours represent the relationship between the sound units and the phonics. The colours represent how these words should be encoded (and spelled), and decoded (and read) until the reading and spelling are automatic.
Stages 1 & 2 colours represent phonemes.
Stages 3 - 6 colours represent large sound units such as prefixes, suffixes, and syllable units (or spoken sound units) etc.

Speech sounds linking to letters or orthographic mapping, is what the colour coding represents.

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