Download the attached EX4 file to your experts\indicators folder and give it a whirl.
It has two modes: Monthly and Weekly. If you set the "Weekly" option to false, which is the default, then it will color the bars differently for each month. Since MT4 has a limit to the number of colors that can be used simultaneously, this indicator uses six colors for January through June, then repeats the same color sequence again for July through December. You can customize the colors to your own liking.
I need some input on the weekly. Currently, it colors each week of the month a different color, for up to five weeks per month (a couple months out of the year have five weeks, the rest have four). If the month ends mid-week, then the bars will change color mid week as well. For example, December 2008 ended on Wednesday - so the color changed on the next day to reflect the new month of January. My guess is that isn't quite what you would want. I think you would want five-day periods colored differently, regardless of which month or partial month they cover. Give me your thoughts on that and let me know what you think.
I can make it color every Monday through Friday sequence of days a different color (up to six or so, then repeat the colors). I think that is more likely what you want.