While explaining colors and how they look might be very difficult, you can help them understand colors using their other senses.
Using touch to describe colors.
Have the person hold certain objects while you tell them what color it is.
It might be helpful to consider using objects that are almost always a certain color.
Have the person hold different pieces of wood, touch the bark of a tree, or touch dirt on the ground, and explain that these things are all brown.
Give the person some leaves or blades of grass to hold, and explain that these are green.
Green feels like the alive parts of plants, because when plants are green that means they are alive.
You can even give some dead leaves and explain the difference between green and brown.
Using sounds could describe colors.
Certain sounds can definitely be associated with certain colors.
Explain that sirens should make them think of red, because red is a color used to get people’s attention and many fire trucks and police and ambulance lights are red.
The sound of running water, especially a stream bubbling or the ocean waves crashing, should make them think of blue.
A sound for green could be the rustling of leaves.
You can also try to describe how the colors make you feel emotionally.
If they are blind from birth they won't be able to see the color in their head, however it gives them associations to use for when someone uses the name of the color.
Hope it helped.
P.S: I totally got help from the Internet for this answer!