First of all, there are gray , white and black colour in picture a,
Moreover, red, yellow and red colour are used and thy all belong to same valus, red and yellow which they are at same level of chroma and they are the adjacent to each other on the colour wheel so the color theory applied is analogic .They create harmonious combinations because they are relatives and have little contrast
First of all, th colour theory is Mono-chromatic because there are only one colour, pink,but Using different shade, tint, or tone of pink to produce three diiferent colour.
Gray pink, pale pink, coral pink are all come from on color, pink, but they in different shades (darker) and tints (lighter) of the same hue. The result is a more subtle effect due to the lack of contrast.