According to Microsoft TechNet:

Disk displays the total current I/O in green, and displays the highest active time percentage in blue.

Source

Notice that this corresponds to the Disk Activity list on the left (might need to expand it), which shows "XX KB/sec Disk I/O" and "XX% Highest Active Time". So, I interpret this as green being both reads and writes, and blue also being reads and writes, but showing the current value of the green line divided by the maximum value of the green line, as a percentage.

Admittedly my source is for Windows Server 2008, not Windows 7.

According to Microsoft TechNet:

Disk displays the total current I/O in green, and displays the highest active time percentage in blue.

Notice that this corresponds to the Disk Activity list on the left (might need to expand it), which shows "XX KB/sec Disk I/O" and "XX% Highest Active Time". So, I interpret this as green being both reads and writes, and blue also being reads and writes, but showing the current value of the green line divided by the maximum value of the green line, as a percentage.

Admittedly my source is for Windows Server 2008, not Windows 7.

The orange line shows I/O for a specific task, enabled by checking a box in the "Processes with Disk Activity" list on the left (might need to expand it also).