Possibly normal.
It could be a type of variegation --> you're the lucky creator of a new variety!
Possibly too much sun.
Xanthophyls which cause yellow colors, protect the photosynthetic apparatus. A. palmatum 'Orange Dream', for example, does this. It's leaves are a nice limey-green color in shade and turn toward yellow with more sun exposure. If this is the case, the yellow color should be fairly rich.
Possibly a mineral deficiency.
Magnesium is at the center of chlorophyll - a pinch of Epsom salt (alternatively you can make a solution for a foliar spray that will act more quickly but won't be as long lasting as when adsorbed though the roots). Iron deficiencies usually affects yellow leaves with green veins (chlorosis). Iron sulfate applied in the same fashion will restore the green.
Possibly root anoxia
Roots need oxygen to do what they do. When they don't get it, they dump an ethylene precursor into the xylem sap. Among other things this may cause the foliage to go through the motions like it is fall already. Most likely remedy is to just stop watering so much! Otherwise, repot as the soil/substrate you are using is too fine grained to admit enough air.
It would be most fascinating to learn that you've applied for a plant patent.