Your Chromebook picks a color from the wallpaper and applies that as a theme for the launcher. Here is how to stop this and use a neutral color instead.
Note: This feature is currently not available in the Stable channel. I am writing this from the Canary channel of Chrome OS. Things move at the Canary speed around here.
Select a Neutral Theme for Chrome OS
Click the down arrow next to the theme selector drop-down.
Next, select Neutral from the Color mode screen. This will use a neutral color as the name suggests.
Select Neutral and you are good to go. Your Chromebook will no longer pick a color from your wallpaper and apply it to the launcher.
Questions? Comments? Let me know in the comments section below.