First, upload your design to Cricut Design Space. If you are totally new to this, I have a tutorial on how to upload your design - File upload to Cricut Design Space
On the right side of the design space, you will see the layers of all the elements of the design. In this example, I use a box template. Click the object you'd like to change the color.
Then click "Linetype" on top left of the design space, then pick the color you prefer.
You could also play with lighter colors when you click the "advanced" under the basic color options.
Send me a message or comment if you have questions. I'll be happy to help :)