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In QGIS 3.16.2 (same on 3.16.0) on Win 10, I created a custom function get_color to get a feature's color by layer name\id and feature ID as proposed in this answer by @Dror Bogin: https://gis.stackexchange.com/a/383753/88814

The expression was successfully created as it appears under custom expressions. However, I'm not able to get the function to run - using this data (project + layers). I try to define the color of the points as the color of the nearest polygon, setting the point's color to data driven override with this expression: get_color('Buildings',array_first(overlay_nearest('Buildings',$id))). This returns an error: Evaluation error: $id. See my screenshot: enter image description here

Using array_first(overlay_nearest('Buildings',$id)) however works fine, it returns a feature no. So $id seems to be a problem together with the custom function get_color in my case.

What is wrong - is there some setting I have to change? I restarted QGIS as well as Windows.

That the solution in principle works is demonstrated by @Dror Bogin who used the very same data as me (linked above) on his Win/QGIS 3.16.1 successfully, see his screenshot, where I get the error mentioned:

enter image description here

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After trying it on another Windows machine and playing around with the settings, I found the problem: the color of my polygon was defined as a categorized style with $id as the value to create the classes for differenty symbol colors. I changed this to an attribute value fid:

enter image description here

Like this, it works perfectly. On the point layer, the expression still is get_color('buildings',array_first(overlay_nearest('buildings',$id))), thus with $id, not with 'fid'. For some reason, the custom function had a problem to refer with $id to the polygon layer that itself used $id to set the color for the features.

As @Dror Bogin mentioned in his post cited above:

the function currently only supports single, graduated and categorized symbol types, and not rule based.

Thus this makes clear why $id did not work.

It would be great to see this custom function become part of the official QGIS code as a new expression.

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    It's probably possible, but I wrote a relatively basic function that uses the layer's attributes and doesn't even support rule based symbology because of this reason (rules and expressions were a hassle to parse and I wanted to do a quick and dirty function)
    – Dror Bogin
    Jan 8, 2021 at 11:01
  • It's quick but not so dirty ;-) OK, that answers also my question - in fact, you mentioned it already in the other comments yesterday. I should have been more attentive.
    – Babel
    Jan 8, 2021 at 11:02
  • Hi @Babel, I had exactly the same experience re using $id (it messes up the colours) as opposed to an attribute defined to have an ID. But I can't work out why. As you say "Thus this makes clear why $id did not work", would you be able to explain a bit more in detail why this is the case? But, or some expected behaviour?
    – puntofisso
    Aug 16 at 10:35
  • I'm quite busy right now and the problem appeared more than two and a half years before, so I don't remember the details. Sorry for not being able to give you an answer - maybe someone else will he able to do so.
    – Babel
    Aug 16 at 11:13

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