In the toddler stages, children have still-developing eyesight. Bright colors are more comfortable than pale colors for them to see at this stage. Bright colors also help little kids to see objects because the bright colors are contrasted against warmer shades, which are harder for them to see. Of course, parents can get involved by teaching their kids the names of the colors so that their child can identify colors by name.
And, lastly, kids often associate colors with objects. Thus, they would call their truck “yellow” instead of “truck.” They relate to objects in their world by color.