What aspect of boro makes it difficult to connect certain colors?; Is this the same aspect that make certain colors more difficult than others to use in general?

It comes down to the minerals and amount of minerals added to the glass.  The more saturaged with minerals, like black takes a lot of heat to move it.  It has a lower visocity.  But they connect fine

Sometimes for instance, if you have lined tubing and one side is all dark, and the other side is light, it will kinda pull to the dark.  It is harder to work, but works when you know how.