//only 1st time at init
memcpy((unsigned char *)temp_pal32out, (unsigned char *)0xDF80, 32); //can be any addr (ROM or RAM), just need 32 bytes full of {0} (=black), can also be done with memset() or if ram is fully cleared (depend on CRT0) no need to add this line

memcpy((unsigned char *)temp_pal32in, (unsigned char *)my_pal, 32); //copy your pal data in ram var before calling the function

SMS_displayOn(); //screen must be "on"
fade_function_ASM(); //call the function
...rest of your stuff here...