How to set label font color using data defined property for PyQGIS 3?

Question

Set label font color using data defined property in PyQGIS 2 is well established, e.g. in another thread

However, PyQGIS 3 completely changed the way this works with no well documented examples. So, how to setup labels whose colors are based on data defined property. Here is my initial try, the label text show up, but the label's font color is not changing as i specified to be dependent upon the 'color' field values:

prop=QgsProperty()
prop.setField("Color")
pc=QgsPropertyCollection('mycol')
pc.setProperty(0,prop)
pal_layer=QgsPalLayerSettings()
pal_layer.setDataDefinedProperties(pc)
pal_layer.fieldName="value"
pal_layer.enabled=True
labeler=QgsVectorLayerSimpleLabeling(pal_layer)
layer.setLabeling(labeler)
layer.setLabelsEnabled(True)

Answer 1

You are setting the size property to take ownership of the field "Color" with this line:

pc.setProperty(0,prop)

The 0 key value is for size, 1 is for bold style etc. The value you want is 4:

pc.setProperty(4,prop)

This will set the colour property to take the field as its expression:

Answer 2

as I do not have enough reputation, yet, to add a comment to the answer from Joseph, I have to add a new one. As far as I understood, the intended use of QgsPropertyCollection.setProperty() is to use an enumeration value casted to integer (see the setProperty documentation).

Therefore, I think the ultimate solution in this case would be something like:

pc.setProperty(QgsPalLayerSettings.Color, prop)

The list of available entries for the QgsPalLayerSettings is given in the QgsPalLayerSettings.Property enum here.

I hope this helps.