Shade

• The most common reason for leaves to be lime-green instead of yellow is not enough sun. Plants need to have at least half a day's full sun for the color to develop.
  
  

Pigments Involved

• Yellow leaf color is due to accessory plant pigments other than chlorophyll called carotenoids and anthocyanins. These are yellow, orange and red in color and usually masked by chlorophyll's green color. More pigments are produced in sunlight to shield the leaves from too much sun.
  
  

Seasonal Changes

• Andrew McIndoe in "Shrubs" notes that golden barberry leaves are brightest yellow in new growth but turn greener later in the summer.
  
  

Mistaken Variety

• Several non-yellow varieties of Berberis thunbergii exist. Perhaps the plant was mislabeled before it was sold. Check other growth characteristics of Berberis thunbergii Aurea to see if they match your plant.