So I still get some rare errors when the process is reading the array, despite that the security is set after the array update in the noteOn, meaning the array has not finished updating when the process begins... That's bad... @Christoph-Hart How can I secure this?
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May I come with my solution? I just have a loop for pev/next so you can continue forever, forever, forever, for... 
@Christoph-Hart What if I update the array in the noteOn, before the condition (isPlaying) that allows the processBlock to read it? Will the array be ready for the process function in time?
note on:
then in the process:
Safe enough?
I don't mind updating the array after a note is played...
@ustk You also need to reset the key colors before the if statement in the for loop with an alpha colour value of 0
@Jerems134 It is the entire for loop you need to move inside 
@Jerems134 nope it is not what you did, you closed the callback right before reg i;
Also you need to reset the previous colour for all keys before applying the new one (especially if the range is not the same). Use any colour with an alpha value of 0
@Jerems134 sorry you already have sample map selection in a callback so just place your code after that, no need for another panel…
Add a hidden panel on the interface. Wrap your key colour code in the panel callback.
Don't forget to enable saveInPreset so when you change preset the callback is called
@Jerems134 It is not the SampleMap you need to check but the sampler itself Sampler1.asSampler() assuming your sampler is called "Sampler1"
Perform this each time you load a new sampleMap