Getting color of feature using PyQGIS

Question

QGIS offers the @symbol_color-variable in some places. But it is missing at various places of data-defined-override/field-calculator dialogues.

I tried to create a own function, but it returns wrong, varying values every time the feature is rendered again (I think, because i work with "defaultSymbol...").

How can I get the real color of every feature by a function ?

(In my case I try to use the result of the function as a data-defined color-override in a 3d-map-view with the goal, that 2d-feature color matches 3d-feature-color)

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom', referenced_columns=[])
def symbol_color(layername, feature, parent):
    layer = QgsProject.instance().mapLayersByName(layername)[0]
    symbol = QgsSymbol.defaultSymbol(layer.geometryType())     
    return (symbol.color().name())

Answer 1

The following code gets the colour of the category that the feature is symbolised by (as it appears you are using categorised symbology in your screenshot) and constructs a dictionary of category values and colour codes. You can then loop through your features to get each one's own colour and add it to the attribute table.

lyr = iface.activeLayer()

r = lyr.renderer()

# get the categories from the renderer
cat = r.categories()

# construct a dictionary of category values and their symbol fill colours
colour_dict = {}
for c in cat:
    # get the category value to use as a key in the colour_dict
    category_value = c.value()
    
    # get the category's symbol 
    sym = c.symbol()
    
    # get (first) symbol layer - it may be a different layer, depending how complex your symbology is
    symbol_layer = sym.symbolLayers()[0]
    
    # get fill colour object from the symbol layer
    col = symbol_layer.color()

    # get the hex and rgb representations of the colour object
    hex_code = col.name()
    rgb = col.getRgb()
    
    # populate the colour_dict
    colour_dict[category_value] = {}
    colour_dict[category_value]['hex'] = hex_code
    colour_dict[category_value]['rgb'] = rgb
    
    print('category:', category_value, 'hex:', hex_code, 'rgb:', rgb)

Output:

category: category 1  | hex: #3d31e7  | rgb: (61, 49, 231, 255)
category: category 2  | hex: #e0675e  | rgb: (224, 103, 94, 255)
category: category 3  | hex: #d647cc  | rgb: (214, 71, 204, 255)
category: category 4  | hex: #83e3ec  | rgb: (131, 227, 236, 255)
category: category 5  | hex: #1de72e  | rgb: (29, 231, 46, 255)

Then you can loop through your features and get the colour for each one individually, perhaps add it as an attribute like so:

# the field that the categorised renderer is using
category_field = 'category'

# add a new field to store the colour
field_name = 'colour'

if not lyr.isEditable():
    lyr.startEditing()

# add the colour field
lyr.addAttribute(QgsField(field_name, QVariant.String, len=0))
lyr.updateFields()

# get the index of the new field
fields = lyr.fields()
idx = fields.indexFromName(field_name)

# add the colour of each feature to the new field
for f in lyr.getFeatures():
    # get the feature id
    fid = f.id()

    # get the category value of the feature
    category = f[category_field]

    # get the feature (hex) colour from the colour dictionary
    feature_colour_hex = colour_dict[category]['hex']

    # insert the colour in the field
    lyr.changeAttributeValue(fid, idx, feature_colour_hex)

# save the changes
lyr.commitChanges()

If you are not using a categorised renderer, let me know and I will update my answer accordingly.