Not sure which verison of Word you are using, but the following works in 2008 and 2011 version.
In Word, select the text you want to highlight and under the Format menu (at the top of your screen), select Borders and Shading ...
and then select Shading from the three tabs at the top of the box that appears.
You will see quite a few more colors, but you can also click the button More Colors and there are an infinity of shades you can create and use, for instance with the color wheel.
On the lower right of this Borders and Shading box select Apply to "text" in the drop down selection. The selected text will be highlighted with your new color.