@rasika The text associated to each hoverbox is in fact the trace name. By default, the line_color/marker_color is inherited by the trace name color to help users identify each scatter trace by color. You can change only bgcolor and font_color.
Note that hoverlabel is a property not only for go.Layout, but of go.Scatter, too.
Just print(help(go.Scatter.hoverlabel) and try to set its keys. You’ll see that the trace name color is unchanged after each new value you assigned to a hoverlabel key.