The eye has "rods" and "cones". Rods see in B&W (grayscale), and "cones" see in color. We have three types of "cones". Each see only one color: blue, green, red (it's actually more complicated, but they are defectively three types) When the green and red cones activate, your brain interpret yellow. And the other colors are also combinations. Take a look at https://en.wikipedia.org/wiki/Cone_cell

Dogs have "rods" and only two types of "cones": blue and yellow https://en.wikipedia.org/wiki/Dog#Senses They also interpret combinations, like us, but have less info to combine.