Some parents become concerned that their pre-school child may be colour blind. This concern may stem from their child being unable to name colours accurately, showing confusion about the names of the colours and saying the wrong colour name. THIS IS NOT COLOUR BLINDNESS. This is a language issue. In other words the child is still learning the names of colours.
Colour is a very complex concept. Allow yourself to think of all the shades of red that there are. And they are all called red. Think of all the shades of blue that there are and they are all called blue. And so on, with other colours. Then there are those more obscure colours with different names. It takes a lot of learning. A lot of people naming for them, so they learn. A lot of children can't name colours accurately until they are 4 years old. This is normal. Some children name colours earlier. Some children name shapes earlier.
If you are still concerned that your child is colour blind then there is a simple way to find out. Most colour blindness tests involve naming numbers and letters hidden in coloured dots. So this is not helpful for a young child. Some have pictures which may be easier, but still involves language concepts.
Can you child match colours? For example, there are puzzles where round pieces are placed on top of a hole which is the same colour as the piece. Or you can make a matching colour board, by having a number of coloured squares or circles on a board and the same number of squares or circles with exactly the same colours. Observe how your child matches these. You need to know that they can actually match. Can they match pictures and shapes? Then can they match colours?
Within other activities, find places for sorting colours or for finding the same colour. See how the child gets on with this and see what you observe.
The most common colour blindness issue is that of red/green. Notice if the child can match all the other colours accurately and yet consistently mixes red/green, sometimes getting them right and sometimes not, or even putting them off the board as they don't know where to put them. Then there may be something to investigate further. If difficulties with matching colours continues into school years, where the child can match pictures and shapes, then a check up is warranted.
Observation can give you lots of information. Observe in many different situations.