You need to use DataTemplate and change the text Foreground property, this is one sample for GridViewColumn.
Check on DataTemplate here: Data Templating Overview
<GridViewColumn DisplayMemberBinding="{Binding Path=FirstName}" Header="First Name" Width="1000">
<GridViewColumn.CellTemplate>
<DataTemplate>
<TextBlock x:Name="Txt" Text="{Binding FirstName}" Foreground="Purple" />
</DataTemplate>
</GridViewColumn.CellTemplate>
</GridViewColumn>