There were two mistakes in this Granny Square, one is a printing error and the other was mine. It comes from “100 Bright and Colourful Granny Squares to Mix and Match” by Leonie Morgan on page 46.
In the foundation row, the written instructions are given as “ch 19 (18 sts + 1 turning chain)”, however, the project requires 21 sts as seen in Row 2 and the graph. So, the foundation row should read “ch 22, (21 sts + 1 turning ch).
My mistake was to work the entire first row in colour A before introducing colour B and following the chart. I should have introduced colour B and begun following the chart from the first row after the foundation chains.