ArrayList<ItemStack> drops = target.onSheared(itemstack, entity.worldObj, (int)entity.posX, (int)entity.posY, (int)entity.posZ,
EnchantmentHelper.getEnchantmentLevel(Enchantment.fortune.effectId, itemstack));
Shears call
IShearable#onSheared
, and sheep implement that interface. So go to
EntitySheep#onSheared
and look how they figure out the color of the sheep.