Homing Y after finishing prints and 2 color models print problem

[Seriousmath]

Good morning all,

My name is Mathieu, French I am 40 years old and I just bought an A20M last week.
This is not my first machine, it was 3 years that I used a JGaurora A5.

I come to you today because I have 2 small problems that I cannot resolve despite my research and skills in the field.
My A20M has a GT2560 V4.0 motherboard but I still managed to get it to work with a CF Marlin 1.1.8 + TMC2208 + BLtouch intended for a V3 motherboard.
The printer is 100% functional and UBL mode activated.

1 / Here is my first problem which must find its answer in the Marlin firmware:

When I finish a print from the SD card, the printer retracts the filament well, it goes well to the coordinates defined in my end Gcode but, once the heating plate has been advanced, it starts back to Y0.
Looks like a G28 Y is present in the machine's internal settings.
I tried looking with Arduino IDE in the code but my skills could not find the problem.

2 / Let's talk about two-tone prints and slicing or perhaps purging:

My prints are very correct but when I use the mix for two-color models, I notice that the lightest color tends to darken during the first few millimeters of printing. However, under Cura I use a 40mm x 45mm3 purge tower. I use as much or more plastic to purge as to print the model and yet I see unsuccessful areas at the end of printing.

I know that this process is not a real Archimedes screw type extrusion, the colors are side by side and that it generates "mixing" concerns but why in some places the colors are perfect and not in others?
If there are few light parts to print on the layer, they will be much darker.

For example I printed an articulated lizard in black and white.
The black is perfect, no trace of white or variation in shade.
On the other hand for white, the first millimeters are gray, then perfectly white and then finally, gray returns.
Possible to put photos if needed.

So much for the little problem I have with this printer after 1 week of use, it must be said that owning a JGaurora A5 is forging 3D printer lol.

Thank you in advance for your welcome and your possible answers.