There is no way to specify the position of an external label. (At some point, we hope to provide some support for this.) We didn't provide an xlabelfontcolor attribute as this can be handled via HTML-like labels. Indeed, since you want the external label centered above this node, this can also be used to solve your first problem. For example,
a [shape=none label=<<TABLE BORDER="0" CELLBORDER="0" CELLSPACING="0">
<TR><TD><FONT COLOR="red">0.19</FONT></TD></TR>
<TR><TD CELLPADDING="6" BGCOLOR="goldenrod2" PORT="p1">would</TD></TR></TABLE>>]
b -> a:p1:n
The PORT attribute is used to get the edge to end at the yellow box.