Hi, right now there's no UI to change the color. However, if you are intrepid enough, you can try modifying attribute Color3 in the BarStyles element in the Gantt chart file.
Open the Gantt chart file with a plain text editor such as Notepad. Look for the following line:
<BarStyles Type="0" Color="-16776961" Color2="-16777216" Color3="-16776961" />
In the example above Color3 is -16776961, which in hexadecimal is FF0000FF. This is in the ARGB format. The left most two digits are the Alpha channel. Leave it as FF. The other three pairs from left to right are red, green and blue respectively. If you don't have a convenient graphics tool to find out the RGB values you want, you can use Word or Excel. Just pick any formatting tool in a Microsoft Office software that uses color.
Hope the above helps.
PS: Modifying a Gantt chart file directly can cause you to lose data if you are not careful. Always make a back-up first before you modify it manually, at your own risks.