I tinkered around a bit, now it should work.
You had to go with the font-size of the diacritic over 20px, to change the color (don’t ask me why, I don’t know). It works now.
Put that in your HTML-Code:
ผล<span class="highlight">อ<span class="diacritic">็</span>ย</span>
And put that into your styling:
.highlight
{
color: blue;
}
.diacritic
{
font-size: 22px;
color: red;
}
It should look like that now: