I have made a working modification for a 10 day color gradiant (yellow-to-red-shift) with a simple case setup, using elseif.

-Colors in rewrites.css could use a little less saturation.
-A more elegant solution with concatenation would be nice.
Basically something like...

$cssvar = '<div class=\"milestone important\"';
for($countervar=0,$countervar<10,$countervar++) {
echo " $cssvar.$countervariable.'>'; }

2 files modified
rewrite.css
view_milestone.php

rewrites.css
---------------------
.important {
border-color: red++++;
/* background: #FFFAFA; */
background-color: #FF5500;
}
.important0 {
border-color: red+++;
background-color: #FF7700;
}
.important1 {
border-color: red++;
background-color: #FF9900;
}
.important2 {
border-color: red+;
background-color: #FFBB00;
}
.important3 {
border-color: red;
background-color: #FFDD00;
}
.important4 {
border-color: yellow++++++;
background-color: #FFFF00;
}
.important5 {
border-color: yellow+++++;
background-color: #FFFF22;
}
.important6 {
border-color: yellow++++;
background-color: #FFFF44;
}
.important7 {
border-color: yellow+++;
background-color: #FFFF66;
}
.important8 {
border-color: yellow++;
background-color: #FFFF88;
}
.important9 {
border-color: yellow+;
background-color: #FFFFAA;
/*weakest important value Green=-#1A,blue=-#14*/
/*prior value FFFFCC*/
}

view_milestone.php
---------------------
<?php
add_stylesheet_to_page('project/milestones.css');
?>
<?php if($milestone->isCompleted()) { ?>
<div class="milestone success">
<?php } elseif($milestone->isLate()) { ?>
<div class="milestone important">
<!-- *****OPEN******** Addition of importance ranking -->
<?php } elseif($milestone->isToday()) { ?>
<div class="milestone important0">
<?php } elseif($milestone->getLeftInDays()==1) { ?>
<div class="milestone important1">
<?php } elseif($milestone->getLeftInDays()==2) { ?>
<div class="milestone important2">
<?php } elseif($milestone->getLeftInDays()==3) { ?>
<div class="milestone important3">
<?php } elseif($milestone->getLeftInDays()==4) { ?>
<div class="milestone important4">
<?php } elseif($milestone->getLeftInDays()==5) { ?>
<div class="milestone important5">
<?php } elseif($milestone->getLeftInDays()==6) { ?>
<div class="milestone important6">
<?php } elseif($milestone->getLeftInDays()==7) { ?>
<div class="milestone important7">
<?php } elseif($milestone->getLeftInDays()==8) { ?>
<div class="milestone important8">
<?php } elseif($milestone->getLeftInDays()==9) { ?>
<div class="milestone important9">
<!-- *****CLOSE******** Addition of importance ranking -->
<?php } else { ?>
<div class="milestone hint">
<?php } // if?>

This is a nice idea, thanks. So do I simply add the text to the files you list or is there something more to do?