|The colours don't match|
|The colours mix|
Colour mixing and matching is all just a numbers game. In the first photo the bag and the shoes look great together as they are both blue reds. In the second the shoes are orange red and the bag is blue red so the colours fight. In the third I added two more reds that fill in between the orange red and blue red and now the colour combination is monochromatic. Think about this when you use multiple shades of a colour of yarn in a single garment or when you want to mix your knitting in with other items in your wardrobe.