Feature colour is lost if colour is adjusted to Colour by Label and then reverted

There seems to be a slightly annoying change in behaviour when switching between feature rendering styles.

In 2.10.5 (and before), if the user enabled 'colour by label' and then after pressing OK, disabled 'colour by label', the original colour would be recovered. This was a side effect of remembering the base colour (as shown in the min/max graduated colourscheme).

On develop, when colour by text is enabled, when the dialog is reopened the min/max colours are both grey, and disabling colour by text sets the feature colour to grey.

Ideally, the old behaviour should be restored - if not, then the colour should at least be reset to a sensible default (e.g. the colour generated from the type string).