Yes! You can ask for minor tweaks and changes to the winning design during the project wrap-up process - as long as they are within reason. Making a few adjustments to color, font, and other small changes to the existing design are reasonable requests.
If you award an entry with the expectation that the designer will make significant changes to the design, you may need to discuss additional compensation for the time necessary to complete those changes. If enough additional work is needed, you can negotiate an amount for the extra work with the creative.
Rest assured that we don’t pay the winning creative(s) until you receive and approve the final files.
For a complete explanation of how to request changes during the project wrap-up process, check out this awesome illustrated guide for design wrap-up or this awesome illustrated guide for naming wrap-up.