I was just going to reply with how you can do it with slicing/CSS outside of flot, but your way sounds much better:
You should be able to use css to change the style of the tick labels. They all have the class "
tickLabel", so you can use jQuery to select the ones you'd like to change. I did this recently, and used Firebug to figure out what order the tickLabels were in (x-axis and y-axis ticks all have the same class). I used jQuery to grab those (6 in my case) ticks and change their color:
var ticks = $(".tickLabel").slice(0,6); //get first six tickLabels
ticks.each(function(i) {
$(this).css("color", "#ff0000");
});