What do you mean? You can change the font color with the "color" attribute in CSS or in the "font" tag in html. You know you can't use a font in a web design if the client doesn't have the font on his/her computer, right? When you say to use a font, it just puts a reference to look for that font. If they don't have it, it will use another.
If I've got this right, you are using text in an image editing/manipulating/creating software and you want to change the color? If I've got that right, it depends on the software.
Now a days usually font properties on a website are defined by a CSS file. Simply edit the cascading style sheet for the website. If you are referring to changing the color in a graphics or other type program, well that depends on the program.