It is hard to tell just when the color was discontinued as the only good source we have on this are the Confidential Bulletins that identified color & trim combinations dealers or distributors could order from the factory.
Carl Spencer picked the color because it was one people would notice; it stood out from the crowd. This was important at 1951 Introduction time because defense work done at Willys-Overland limited availability of the 4-cylinder engines in particular. Spencer wanted what could be built initially to be noticed with minimum connection to the idea people were seeing the same car at various places around town.
I know I am missing some Confidential Bulletins from the later part of the 1951 model year that would include availabilities for colors and trims (which would be a mix of colors for new production as well as finished cars in inventory). However, the equipment, color & trim listing from January 26, 1951 does not list Aloha Green as available at that point. Normally when a color was discontinued, there was no announcement from the factory; any dealer orders for a discontinued color, trim or item were just not accepted for production and fulfillment.