Bug explanation
When creating a datagrid with an editable combobox, its text cursor caret will not take up the normal foreground color:

As a solution I added CaretBrush to the MaterialDesignDataGridComboBoxEditableTextBox style in the source:
<Setter Property="CaretBrush" Value="{Binding RelativeSource={RelativeSource AncestorType={x:Type FrameworkElement}}, Path=(TextElement.Foreground)}" />
resulting:

Version
Latest master