Combine Hairs is a new tool which combines several hair particle systems into a single one. This is useful because the Make Hair tool tends to create many different particle systems, and that happens because all strands in a given particle system must have the same number of vertices. The Make Hair tool has a resize option, but it may not be obvious which is the right size to resize to until after the particles have been created.
Here is Toulouse hair with three different materials, before and after particle hair has been made.
Everything looks nice in the viewport, but we see that a lot of different particle systems have been created. To clean things up, we now want to combine the particle systems into a few ones.
At the bottom of the Hair section, press Combine Hairs.
A dialog appears where we can select which particle systems we want to combine. The Hair Length parameter specifies the number of vertices in the strands (the number of segments plus one). The numbers in parantheses is the hair length of the corresponding hair.
Since all hairs have the same length (24), we set the Hair Length parameter to 24. To combine all green hairs, filter the list with "green" and press the All button. The green particle systems are now combined into a single one, called Hair_24_Green. Repeat the procedure to combine the red and blue hairs, too.
Now we only have three particle systems, one for each material. There is no visible change in the viewport.
We use the tool again to combine the three particle systems into a single one. We specify that the strands have ten vertices (nine segments).
And we now have a single, green, particle hair. If we want to change the material, it can be easily done in the particle settings
