That answered it, brilliantly, ta, in a nutshell. But no custom code involved.
I had copied what was offered as the color array by your new colorPicker into the color field for the “screen”. That was a 4 element array. Culling the 4th element, resynching, etc., got this dummy experiment going. Different font, but so what.
So the error propagated from the colorPicker-- I can confirm that between versions 2021.1.0 and 2021.1.0 this new widget now pops up (colorChooser, palette window, doesn't pop up - #2 by TParsons), but it doesn’t “push” a colour value into the field/for the parameter in question. Instead, it seems that the value it produces has to be manually copied back into that field. Anyway, how it works isn’t intuitive. But what it produced, anyway, in my case, was a 4-element array. You also, of course (?), have to prepend what the colorPicker produces with a “$” sign; the new colorPicker just returns a naked 4-element array. I did that already, to get it working by Python. But this wasn’t enough to get it working by Javascript, on Pavlovia.
But now manually clobbering the colorPicker value into a 3-element array, as you indicate, and we’re dancing. Ta. Now not only via python, but also via Javascript, on Pavlovia.
Just to round off, re my last question, above, so a local “lib” directory of your .js library is no longer to be expected upon creating the PsychoJS experiment?
If so, does that mean, however, that we still have to create a “resources” folder in which we duplicate all of our condition files, media, etc., for running online versions in 2021?