Question

is it possible to specify the color for the row selector in silverlight grid

Answer 1

Yes but you need to copy the control template for the DataGridRowHeader control and place it in a Style object in a resource:-

<UserControl.Resources>
  <Style x:Key="CustomRowHeader" TargetType="DataGridRowHeader">
    <Setter Property="Template">
       <Setter.Value>
          <ControlTemplate TargetType="localprimitives:DataGridRowHeader">
            <!-- Copy of the rest of the standard controls template -->
          </ControlTemplate>
       </Setter.Value>
    </Setter>
  </Style>
<UserControl.Resources>

<DataGrid RowHeaderStyle="{StaticResouce CustomRowHeader}" ... >

Now you can fiddle around with the color value and pretty much anything else that is used to render the row selector.

You can probably do this with Blend fairly well if you have it and are familiar with using it. I prefer to copy the template from the documentation. See DataGrid Styles and Templates