Colouring points in the same colour as their nearest point/polygon from another layer in QGIS

Question

I got two files, a multi-polygon file with polygons and points (File 1) and a file with points only (File 2).

Our task is to fill the color of the points from File 2 with the respectively nearest polygon or point from File 1.

I use QGIS. I'm almost sure there is an expression or method to do just that!

We also got another line layer that shows the distance between the points and the respectively nearest point/polygon (File 1), so I could technically color it all manually, but I want to learn how to automate my workflow.

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EDIT 1: @Babel’s answer does not work for me so far. Here are the wrong symbols: https://ibb.co/ZXSkk2y My two layers do not have the column "Id" but @osmId in the attribute table instead, but changing the "Id" with "@osmID" doesnt work either :/ https://ibb.co/020zD61 This is my command, which should be correct...: https://ibb.co/Fwy10Xk I also tried to activate the current expression here, didnt work either: https://ibb.co/Y3bz13p And lastly, I should add that that file 1 is a multi-polygon file with polygons as well as points...maybe theres the error?

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EDIT 2: Those are my values from Layer/File 1 (points): https://ibb.co/r2wh0MX and those are my vaules from Layer/File 1 (polygons): https://ibb.co/Df3dfgJ

Both definited by @osmId because there is no column "Id"

Now I want that the points from Layer / File 2 (Kindergarten_Karlsruhe) have the nearest colour either from the point from Layer/File 1 (Krankenhaeuser_Karlsruhe) (points) or from Layer/File 1 (Krankenhaueser_Karlsruhe) (polygons).

Layer 2 (Kindergarten_Karlsruhe) should receive the colour from either of those two geometries (polygons/points) from Layer 1 (Krankenhaeuser_Karlsruhe) which the respecitve kindergarten point is closest to.

Here you can see both Layers again: https://ibb.co/n3T03m2

Now, if type in the formula instead of "Id" "@osmId", there is an error messing appearing... see also: https://ibb.co/12jMZV2

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EDIT 3:
This is correct, I have two (grouped) layers, both named Krankenhaeuser_Karlsruhe, once as polygon and once as points. I renamed both of them and set the colour settings from both again and tried to type in the command - still with an error!
See: https://ibb.co/4ZS08qQ
Error Message: https://ibb.co/vYSGNxn

Answer 1

You can use the new overlay expressions, available since QGIS version 3.16, see Visual changelog. By the way: I'm not sure what you mean that you have "a multi-polygon file with polygons and points", that does not make sense to me, so maybe be more precise. Likewise, you did not include any information how the colors in layer 1 are defined. But anyway, once you understood how the get the color from one layer to another, you can play around with the settings and adapt it to your needs.

So here I show the principles. For demonstration purpose, let's suppose the features from layer 1 have a categorized color, defined by an attribute value, from a color ramp. I used the id as value for the definition of the color and the Viridis color ramp as you can see on this screenshot:

Now, on layer 2 with the points, I set the color to data driven override and select assistant:

In the dialog panel, paste this expression in the source field:

array_first (overlay_nearest('polygon',$id))

So for every point, the expression creates an array of features, ordered by distance (overlay_nearest) from the layer 'polygon' and returns the id of these polygons (id of nearest, id of second nearest, id of third nearest etc.). With array_first you get the first, thus the nearest, polygon id. Every point now has the id of the nearest polygon. We use that and set the same Viridis color ramp to get the same color (the small dotted black line is for demonstration purposes only):

Update: See on my first screenshot that I have 15 classes for colors (1 to 15) plus another one for "other values" (yellow) - that makes 16 together. So be sure to load the valus when defining the colors of layer 2 (below the field where you paste the expression): see last screenshot, double arrow symbol. You could also set manually values from 1 to 15 - or 16, in case the colors don't match to conform with the number from above.

Answer 2

Using @babel's answer I wrote a function to get a feature's color by layer name\id and feature ID.
You are welcome to add it to you QGIS with the function editor.

from qgis.core import *
from qgis.gui import *
from PyQt5.QtCore import QVariant

@qgsfunction(args='auto', group='Custom')
def get_color(layer, uid, feature, parent):
    """
    Gets the main color for a feature, works with 'single symbol', 'graduated symbol' and 'categorized symbol' types.
    Returns a RGBA string.
    <h2>Example usage:</h2>
    <ul>
      <li>get_color('Buildings',47) -> '255,100,46,255'</li>
    </ul>
    """
    layers_names = []
    for ll in QgsProject.instance().mapLayers().values():
        if ll.name() == layer or ll.id() == layer:
            layer1 = ll
    
    iterator = layer1.getFeatures(QgsFeatureRequest().setFilterFid(uid))
    feature = next(iterator)
    color = [0,0,0,0]
    if layer1.renderer().type() =='singleSymbol':
        rgb = layer1.renderer().symbol().color().getRgb()
        color = '{},{},{},{}'.format(rgb[0],rgb[1],rgb[2],rgb[3])
        
    if layer1.renderer().type() == 'graduatedSymbol':
        for range in layer1.renderer().ranges:
            attribute = layer1.renderer().classAttribute()
            value = feature.attribute(attribute)
            return value
            if range.upperValue() > range and range.lowerValue() < range:
                rgb = range.symbol().color().getRgb()
                color = '{},{},{},{}'.format(rgb[0],rgb[1],rgb[2],rgb[3])

                
    if layer1.renderer().type() == 'categorizedSymbol':
        attribute = layer1.renderer().classAttribute()
        value = feature.attribute(attribute)
        catValues = []
        for cat in layer1.renderer().categories():
            if type(cat.value()) != QVariant:
                if int(cat.value()) == value:
                    rgb = cat.symbol().color().getRgb()
                    color = '{},{},{},{}'.format(rgb[0],rgb[1],rgb[2],rgb[3])

    return color

After adding it to your QGIS expression engine (just go to the function editor, create a new file, copy and paste the code and save) you could use an expression like this:

get_color('Buildings',array_first (overlay_nearest('Buildings',$id)))

Note that the $id part is the automatic feature ID in the layer, and not a specific attribute.

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

You can even keep using a single symbol style and just edit the expression for the fill color: