how to insert values to the Datagridcell in a Datagrid

I have created a Matrix kind Datagrid using the following XAML in my application.

XAML

<DataGrid x:Name="Matrix_datagrid">           <DataGrid.RowHeaderTemplate><DataTemplate><TextBlock 
    Text="{Binding RelativeSource={RelativeSource Mode=FindAncestor, 
    AncestorType={x:Type DataGridRow}}, 
    Path=Item.Header}"/></DataTemplate></DataGrid.RowHeaderTemplate><DataGridTextColumn Header="Availability"/><DataGridTextColumn Header="CPU"/><DataGridTextColumn Header="RAM"/><DataGridTextColumn Header="Hard Disk"/><DataGrid.ColumnHeaderStyle><Style TargetType="{x:Type DataGridColumnHeader}"><Setter Property="LayoutTransform"><Setter.Value><RotateTransform Angle="270" /></Setter.Value></Setter>                    </Style></DataGrid.ColumnHeaderStyle>          <DataGrid.Columns>                <DataGridTextColumn Header="Server1"/><DataGridTextColumn Header="Server2"/><DataGridTextColumn Header="Server3"/><DataGridTextColumn Header="Server4"/><DataGridTextColumn Header="Server5"/>           </DataGrid.Columns></DataGrid>

Output

I want to evaluate a Boolean expression in the datagridcell.
for each X and corresponding Y axis , I have boolean expression to be evaluated. 
When the expression is TRUE the cell must display green color and whenFALSE , must display red color. I can achieve this via Triggers.
but how to access these datagridcells ? any ideas ?