Tab notation and note lengths

[Ian Hulin]

I've just read Johan's reply on another thread about tabs and note 
stems, which prompted me to ask.

How do differentiate between crotchets (quarter-note) and minims 
(half-note) lengths in tab and also their dotted versions?  Should lily 
use a white-font-on-black-black background for the strings when it 
represents a crotchet/quarter-note?

Cheers,

Ian