How can I add different colors for the different menupoints?

In the backend there is no option for this, only for the different levels, but with some css code you could do it.

First of the first level menupoints:

.level1 .level1:nth-child(1){
background:green!important;
}

It’s submenus:

.level1 .level1:nth-child(2){
background:green!important;
}

Second of the first level menupoints:

.level1 .level1:nth-child(3){
background:green!important;
}

It’s submenus:

.level1 .level1:nth-child(4){
background:green!important;
}

The first item from them:

.level1 .level1:nth-child(4) .level2 .level2:nth-child(1){
background:green!important;
}

The second item from them:

.level1 .level1:nth-child(4) .level2 .level2:nth-child(3){
background:green!important;
}

The submenus of this second item:

.level1 .level1:nth-child(4) .level2 .level2:nth-child(4){
background:green!important;
}

The first submenu from these:

.level1 .level1:nth-child(4) .level2 .level2:nth-child(4) .level3 .level3:nth-child(1){
background:green!important;
}

The second submenu from these:

.level1 .level1:nth-child(4) .level2 .level2:nth-child(4) .level3 .level3:nth-child(3){
background:green!important;
}

It might looks complicated, but basically you just have to go down in the ladder of levels to your menuitems.