You are right, unfortunately GraphicsLayer has no SelectionColor property.The way to do it is by defining your own symbols with 'Selected' selectionStates.As sample, here is how the X feature symbol is defined:
<ControlTemplate x:Key="FS_SimpleMarkerSymbol_X">
<Path x:Name="Ex"
Stretch="Fill"
Data="M0,1 L1,0 2,1 3,0 4,1 3,2 4,3 3,4 2,3 1,4 0,3 1,2 Z"
Width="{Binding Symbol.Size}"
Height="{Binding Symbol.Size}"
RenderTransformOrigin="{Binding Symbol.RenderTransformPoint}"
Fill="{Binding Symbol.Color}"
Stroke="{Binding Symbol.OutlineColor}"
StrokeThickness="{Binding Symbol.OutlineThickness}"
StrokeDashArray="{Binding Symbol.OutlineStyle, Converter={StaticResource DashArrayConverter}}"
StrokeLineJoin="Round">
<Path.RenderTransform>
<RotateTransform Angle="{Binding Symbol.Angle}"/>
</Path.RenderTransform>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="SelectionStates">
<VisualState x:Name="Unselected" />
<VisualState x:Name="Selected">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="Ex"
Storyboard.TargetProperty="Fill">
<DiscreteObjectKeyFrame KeyTime="0:0:0" Value="{Binding Symbol.SelectionColor}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
</Path>
</ControlTemplate>
