Using the emptyMessage attribute of the list widget you can make simple if statements. Like this:
<ol>
<$list filter="[!has[draft.of]tag[3-act]sort[title]]">
<$list filter='[is[current]tag[feina]][is[current]!tag[completada]]' emptyMessage='<li><$link to={{!!title}}><$view
field="title"/></$link><$list
filter='is[current]has[abstract]'> <em><$view
field="abstract"/></em></$list></li>'>
<li style='color:red'><$link to={{!!title}}><$view field="title"/></$link><$list filter='is[current]has[abstract]'> <em><$view field="abstract"/></em></$list></li>
</$list>
</$list>
</ol>
with the inner list widget if the current list item has the tag feina or doesn't have the tag completada than the contents of emptyMessage is shown, otherwise the one with the color is shown.
There are other ways to do this using either the set or the reveal widgets also, but I think this is the simplest one.