How to extract "derived" attribute values in QGIS (using PyQGIS) into the attribute table?

Question

I know how to extract the geometry data, but I am struggling with the following derived attribute data in QGIS:

Searched a lot for extracting derived data, but only geometry data comes up. Result: I want to fill a new column with the color code.

@symbol_color works fine within the symbology, but it is not usable for the field calculator.

Original data:

Label using @symbol_color

If I can label all the features with the color code, there must be a possibility to save the color code in the attribute table.

Answer 1

There is probably a more direct method, but you can drill down through the layer's renderer to get the categories of a categorised symbology, and subsequently the colours used in their symbol layers.

The categories I used are strings of random integers between 100 and 500.

For each category, you can get the attribute value used to symbolise it with its .value() method.

## get the layer
lyr = iface.activeLayer()

## get the field index of the attribute used to classify the symbology
category_field = lyr.fields().indexFromName('category')

## get the layer's renderer
rnd = lyr.renderer()

## get the categories from the categorized renderer
cats = rnd.categories()

## get the dataProvider of the layer to add a new field in which to store the values
prov = lyr.dataProvider()

## name of field
rgb_field_name = 'RGB'

## if field does not exist, create it
if rgb_field_name not in lyr.fields().names():
    prov.addAttributes([QgsField(rgb_field_name, QVariant.String)])
    lyr.updateFields()
    lyr.commitChanges()

## get the layer's features as an iterator
feats = lyr.getFeatures()

## open an edit session
lyr.startEditing()

## iterate through features
for f in feats:
    ## iterate through the categories for each feature
    for c in cats:
        
        ## get the attribute value used for the current category
        cat_value = c.value()
        
        ## check if the current feature's category attribute matches the category value;
        ## if so, get the fill colour from the feature's symbol
        if f[category_field] == cat_value:
            ## get symbol
            s = c.symbol()

            ## get symbol layers
            sym_lyrs = s.symbolLayers()

            ## get the first symbol layer
            sym = sym_lyrs[0]

            ## get fill colour of symbol layer
            col = sym.fillColor()

            ## get the rgb values of the color
            rgb = col.getRgb()
        
            ## rgb is a tuple, so convert it to a string and assign it to the new field
            f[rgb_field_name] = str(rgb)
            ## update the feature's attributes
            lyr.updateFeature(f)

## save the edits and close the edit session
lyr.commitChanges()

I am not sure how to get the hex color code, but there is probably a way of translating the RGB to hex if you particularly need hex.