Different Color Bars for Flot Categories Bar Chart

When you put your data you must to put the colors inside:

var data = [
    {color: '#ff00aa', data: [[0, 1]]},
    {color: 'red', data: [[1, 1]]},
    {color: 'yellow', data: [[2, 2],[3, 2]]},
    {color: 'orange', data: [[4, 2]]},
    {color: 'blue', data: [[5, 4],[6, 7]]},
    {color: '#000000', data: [[7, 1]]}
];

As is often my recommendation with Flot, drop the plugin and configure it up youself.

// separate your 3 bars into 3 series, color is a series level option
var data = [{data: [[0,1]], color: "red"}, 
            {data: [[1,2]], color: "yellow"},
            {data: [[2,3]], color: "green"}];

$.plot("#placeholder",data, {
    series: {
        bars: {
            show: true,
            barWidth: 0.3,
            align: "center",
            lineWidth: 0,
            fill:.75
        }
    },
    xaxis: {
        // drop the categories plugin and label the ticks yourself
        // you'll thank me in the long run
        ticks: [[0,"Red"],[1,"Yellow"],[2,"Green"]]
    }
});

Running code:

var data = [{data: [[0,1]], color: "red"},
            {data: [[1,2]], color: "yellow"},
            {data: [[2,3]], color: "green"}];

$.plot("#placeholder",data, {
    series: {
        bars: {
            show: true,
            barWidth: 0.3,
            align: "center",
            lineWidth: 0,
            fill:.75
        }
    },
    xaxis: {
        ticks: [[0,"Red"],[1,"Yellow"],[2,"Green"]]
    }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/flot/0.8.3/jquery.flot.js"></script>
<div id="placeholder" style="width:400px; height: 300px"></div>