To get the same colors, you need to set them directly in series options. Then to get in legend only group 1 and 2 add ID's to the series and link next series within the same group. Example: http://jsfiddle.net/3kpTK/2/
series: [{
id: 'g1',
color: 'blue',
name: "group1",
data: [50,20,30,50],
stack: "move"
}, {
linkedTo: 'g1',
color: 'blue',
name: "group1",
data: [10,23,0,52],
stack:"delete"
},{
id: 'g2',
color: 'green',
name: "group2",
data: [50,20,30,50],
stack: "move"
}, {
linkedTo: 'g2',
color: 'green',
name: "group2",
data: [10,23,0,52],
stack:"delete"
}]