You guys are funny. Just because a BTA may not be as colored up as some fantastically perfect sunburst on a retailers site doesn't mean that it is not a sunburst. We (Premier Fish and Reef) have had literately hundreds of sunbursts (still have quite a few) over time. We actually received some of the first splits from Kei years and years ago. Some have been extraordinarily colored up while others have been, meh (usually due to lighting conditions).
What makes a sunburst is a yellow tip, period, end of story. You can have a BTA with rose color and a yellow tip like so:
and it is still a sunburst. How do I know? Because here is the mother:
How do I know the mother is a sunburst? Because I can actually trace the lineage back to Kei's original.
Here is another (crappy image I know but you can see the yellow tips):
and another:
As you can see there are a lot of variations in color pattern, how they bubble but they all have the yellow tips and every picture linked I can trace lineage.
Typically speaking rainbows do not have yellow in them. However, we have seen, if you have a rainbow or rose in with a sunburst the sunburst can stimulate the rainbow or rose to pigment closer to the sunbursts colors (and vice versa). This change is pretty settle though and a sunburst will always have yellow tips.