.green_guys + p {
    /* selects the <p> element that immediately follows .green_guys */
}

.green_guys ~ p {
    /* selects all <p> elements that follow .green_guys */
}

<input name="importantAnswer">
<div class="help-text"></div>
<div class="invalid-text"></div>

<input name="importantAnswer">
<div class="messages">
   <div class="help-text"></div>
   <div class="invalid-text"></div>
</div>


.help-text, .invalid-text {
   visibility:hidden;
}

.input:active +.messages > .help-text {
   visibility:visible;
}

.input.invalid:visited +.messages > .invalid-text {
   visibility:visible;
}

p ~ p:not(.green_guys),
p:first-child:not(.green_guys) {
  text-decoration: line-through; /* or whatever you like */
}