Explain that sirens should make them think of red, because red is a color used to get peoples attention and many fire trucks and police and ambulance lights are red.
Say, When you hear a siren, it is to cause people to be alert and pay attention right away, because there might be danger. Red is like that its urgent and grabs your attention.
The sound of running water, especially a stream bubbling or the ocean waves crashing, should make them think of blue.
Say, blue is calm and nice, like how the sound of water makes you feel relaxed.
A sound for green could be the rustling of leaves, or the chirping of birds. Explain that not all birds are green, but because birds live in the trees, the sounds of birds often make people think of the color green.
Say, When you hear trees rustling and birds singing, that is what green sounds like.
Describe the sounds of a storm as grey. When there is thunder and falling rain, the sky is grey and it makes everything look more grey spacebar clicker.
Say, Storms are grey. The sounds of the loud thunder and rain mean that it looks grey outside, its a bit dark and depressing because the sun isnt out.