It's actually easy enough with a bit of inline CSS - see this simple XML dashboard example, which hides the colour dropdown and sets the colour to black. If you select A, C or D then the colour dropdown is re-shown.
<form version="1.1" theme="light">
<label>ddl</label>
<fieldset submitButton="false">
<input type="dropdown" token="component" searchWhenChanged="true">
<label>Component</label>
<choice value="A">A</choice>
<choice value="B">B</choice>
<choice value="C">C</choice>
<choice value="D">D</choice>
<change>
<eval token="display_colour">if($component$="B", "display:none", "")</eval>
<eval token="colour_name">if($component$="B", "No Colour", $colour_name$)</eval>
<eval token="form.colour">if($component$="B", "#000000", $form.colour$)</eval>
<eval token="colour">if($component$="B", "#000000", $colour$)</eval>
</change>
</input>
<input type="dropdown" token="severity" searchWhenChanged="true">
<label>Severity</label>
<choice value="Info">Info</choice>
<choice value="Warning">Warning</choice>
</input>
<input id="colour_dropdown" type="dropdown" token="colour" searchWhenChanged="true">
<label>Colour Dropdown</label>
<choice value="#000000">Black</choice>
<choice value="#ff0000">Red</choice>
<choice value="#00ff00">Green</choice>
<choice value="#0000ff">Blue</choice>
<change>
<set token="colour_name">$label$</set>
</change>
</input>
</fieldset>
<row depends="$AlwaysHideCSS$">
<panel>
<html>
<style>
#colour_dropdown {
$display_colour$
}
</style>
</html>
</panel>
</row>
<row>
<panel>
<html>
<h1 style="color:$colour$">Component $component$, Severity Level $severity$, Colour $colour_name$</h1>
</html>
</panel>
</row>
</form>
