I have the bar chart below:

![enter image description here](https://i.stack.imgur.com/0VoMc.jpg)

My code:

```
BarChart[{Labeled[{a760, c760, b760, d760}, "760" ], 
  Labeled[{a780, c780, b780, d780}, "780" ]}, 
 ChartStyle -> {Blue, Red, Green, Black}, 
 ChartLegends -> 
  Placed[{"65.5 ps", "274 ps", "16.8 ps", "2040 ps" }, {Right}], 
 BarSpacing -> {0, 1}]

```

The third entry of 780 is missing, so I used `Missing[]`. I want to attach the forth entry 

of 780 to the second entry of 780. The point is that the color of the forth entry must 

remain black because each color indicates a quality such as lifetimes.