A lot of preschoolers are just learning their colors. This can be fantastic as the subject of a bulletin board. Consider the following strategies.
Make a rainbow
A lot of children love rainbows, and this can be very pretty on a bulletin board. You can have the children help paint or color the rainbow on the board. You can also be creative and use something like fabric or even wrapping paper to make up the rainbow. You can label the colors.
Put up pictures of items in different colors
You can arrange the bulletin board into different categories. For instance, you can break it up into six different boxes. In these boxes you can put in pictures of objects that are all of a certain color. You can label the boxes with the name of this color. One great way to get the children involved would be to give them a bunch of magazines and have them cut out pictures of different colors. You can then have them come up and place them into the right category, and you can post them all up together. You can also actually attach objects of certain colors that are small enough to affix to the board. For instance, you can attach crayons of a certain color or a little stuffed animal that is a certain color. You want the children to start to distinguish the colors.
Make it interactive
If you want the bulletin board to be about teaching the colors, then make it interactive. Take a bunch of pieces of paper and attach them to the bulletin board only by the top so that they can lift up like a flap. On the top of them you can put a picture of something that is a specific color and underneath write out the name of the color. The children can then guess the color and then lift it up in order to see the color. If they cannot read then you can do it with them as a class project.
Get the children involved
Many children learn best if they play an active part in creating something. You can have the children each focus on one or several colors. They can draw pictures of items that are generally seen in that color. For instance, you can ask that they draw items that are normally red, and hopefully they will pick apples, tomatoes, etc. You can then post these to make up your bulletin board.
You want the preschoolers to know their colors, and a bulletin board can be a great teaching tool. Consider the above ideas when creating your bulletin board.