Our eyes have three cone cells that detect color. These cone cells detect the colors red, green, blue, known as RGB. The reason we can see colors other than red, green, or blue is because every color stimulates more than one cone cell. Their combined response produces a unique signal for each color.
Light is made of all colors. When a ray of light hits an object, that object's color is reflected and all other colors are absorbed.
Each time light reflects off an object, the reflected light enters our retinas, which have rods and cone cells in them. These cone cells each send off a signal to the brain depending on the color we see. When we see pink, a combination of colors is sent to the brain.
Some people have missing types of cone cells, making them color-blind to those specific colors. One example of this is when people do not have any red cone cells. If they do not have these cells, they will not see colors that have red in them.
Each cone cell in an eye has different photopsins. Every photopsin has a distinct response to light, and thus reacts to color in different ways.