I make the leaves green, I make the dirt brown, I make the blue birds blue, I make all of your childhood dreams come true. What am I?

“Paint”

While paint can also create colors and is used in artistic expression, it doesn't specifically relate to childhood dreams in the same way crayons do, which are often associated with early childhood art activities.

“Grass”

Grass does indeed make leaves green and dirt brown, but it doesn't encompass the variety of colors (like blue birds) and the direct connection to childhood creativity that crayons represent.

“Sky”

The sky can appear blue and is part of nature, but it doesn't make dirt brown or have a direct connection to the colorful artistic experiences of childhood like crayons do.