I have 3 spheres I'm importing via alembic which have 3 vertex colors assigned (one color per sphere). I can access the attribute after an unpack node and assign a color using the color node “material_blue” for example in the group.

How do I make those colors end up on my final mesh and have them blend together ?

Sorry for the noobish question. I also want to export those vertex colors to an alembic and render them in maya or modo.

I think you need to actually assign a material per group. That should create a vertex color in a alembic on output. Just using the Color node is not enough.

I don't even see why you want to go through Houdini in this case. Just import your spheres into Maya or Modo and have at it.

Materials have no effect on alembic export. it will simply add an unusable property. Simply add the color attribute ‘Cd’, either point or vertex. When importing into maya, add -rcs to the AbcImport mel command to import the color set data.
https://www.sidefx.com/forum/topic/38939/ [sidefx.com]

Edit:
according to the last post in that thread, you don't even need -rcs if you are using a modern version of houdini to export.