If you add the following CSS snippet to a CSS file you were importing, you should see the x-axis color change:
.flot-x-axis {
fill: rgba(255, 0, 0, 1);
}
You can also target specific axes by their labels (using the same class-based target). Note that this will just change the color of the major division labels (not the ticks or baseline). To change the ticks, you must setup the axes in flot to use a particular color like so:
$.plot("#placeholder", [...], {
xaxes: [ { color: "red" } ],
});
This is undoubtedly unintuitive, and it would be good to try and correct this holistically. I imagine the way ended up where it is had to do with the slow migration of rendering visuals using SVG that could leverage CSS more easily, and it stopped when the labels moved over to this system.