There are 2 colors to be used: one for the stroke and the other for the fill. Basically, each object supports these parameters.

For changing the fill color of the object:

Select the object and click the Fill color box to activate it. Specify the color and you're done!

For changing the stroke color of the object:

Select the object and click the Stroke color box to activate it. Specify the color and you're done!

For changing the color of the text:

Highlight the text and click the Text color box to activate it. Specify the color and you're done!