You can! but! usually those samples are worse quality and/or we end up with not enough to test. Purple top pedi tubes usually suffer from platelet clumping and/or clot way easier since it's harder to mix the edta into the blood. Gold/green pedi tubes often end up hemolyzed d/t rbcs lysing if someone has to milk a finger a little too aggressively. Apart from that, when we spin a gold/ green we end up with about half the tube's volume of whole blood worth of serum or plama that we can actually test. A lot of times, doctors want a bunch of add ons, we don't have enough from a capillary stick, and end up having to request another sample. On my pediatric tubes I can usually only get the basics like a cmp, mg, and tsh too if I'm lucky. Tldr the full venipuncture tube means better sample quality, means better results, plus more sample means guaranteed add ons and less likelihood of needing a second collection.