I can’t get my Ender 3 pro with version 1.1.8 of Marlin to recognize the M500 command in my Prusa Slicer. I can get a filament change in cura but I really prefer Prusa and it should work but it doesn’t. I tried to update with arduino and got an error message that TH3D never responded to. I can still do multicolour another way but would really like to have some G code to put in my Prusa to do the filament change equivalent of M600.
You don´t need that. You can use the slicer to insert the commands right into the gcode. You printer will pause, you should be able to swap filament and continue with a push on the button.
I have no clue about if the ender printer allows to change filament in pause mode, but on the other side: I never did that automatically. 
I plan to add this functionality to my own printer, but on both other printers I always swap it by hand.
The only thing I recommend is using the “M18 S9999” command. It disables the motor timeout, so you cannot move the print head while changing filament. By default this is only somewhat 30 to 60 seconds, which is stressful. More or less only important when using a direct drive with the extruder motor on the print head.
Ether way, push and hold the extruder button, grab the filament, push it a little into the printing direction to get the tip hot and soft again and then pull it out fast. Now insert the new filament and while holding the extruder button, move it in, until you get the new color out of the nozzle. Give it some extra pushes to get rid of any of the old color fragments and after you remove the stuff below the nozzle you are good to go and can continue the print. Basically it is a filament change like you do it at the end of every roll anyway.
I don’t want to hover over my machine while it’s printing. The M 600 command usually takes the printhead out of the way and gives a beep and it will cool the hot end after a certain period of time. So if you are on hand you can pull out the old colour and thread in the new. If you aren’t you can resume and while the hotend id heating up you do this. I think it even gives you extra time but am not sure.
I have tested my resume function many times and it works well for power failures or if I just shut it off. It resumes usually like nothing happened. It works a little funky with my stop print. It always tries to resume unless I get another gcode running.
Why not use an M0 command? It’s an unconditional stop. Use an M117 command to put a message, like “change filament” on the LCD screen, and M300 commands to make the LCD Screen alert you with sound. For example:
M300 S440 P200
M300 S660 P250
M300 S880 P300
M117 change filament, then click
M0 ; Pause print
How could I test this. Thank you so much for responding.
Easy: copy the italicized lines into a text file on your computer and save it. Check the file’s name. Make sure it ends with “.gcode”. Put it on your micro SD card and print it. You should hear the sound, and see the message on the LCD.
Once you see that working then, in Prusa Slicer, you can go to the layer where you want to change filament, click on the orange circle with the white + in it, select to add Custom GCode, and paste the same lines into the box that pops up.
While in theory I am fascinated with multi-color printing, and I have used it to make signs, I find the idea of dissolvable supports even more interesting. If I invest in the future in a multi-filament print it will be to gain access to dissolvable filament. The challenge is still all of the waste in multi-filement prints when using a single print head. For my purposed, I think a printer with multiple print heads is more interesting.
Yeah, dissolvable materials are interesting. One of the main reasons to build my own multi head tool changing printer.
It also is neat to not being forced to swap filament each time you need a different color. I mostly do practical prints and swapping between white and black depending on the use is not annoying, but time/work I can spare in the future. Still working out the software as Marlin is kind of a spaghetti disaster due to its 8 bit optimizations.
PS. I am the guy with all the FreeCAD suggestions/comments on youtube.
YES! THIS! The ability to use PVA (for PLA), HIPS (for PETG), or some other dissolvable filament as support material would completely change my outlook on using supports. At some point I will look at adding an extruder to my Ender 5. Are you listening Irv (hint, hint)
I’m already up to my eyeballs in this technology without understanding what this conversation is really about.