I'm not a Krita user myself but if you are new to the software then you will need as a minimum a good understanding of using layers and using the selection tools. Then you will know how to select certain body parts and just re-color the parts you want to change. You should be able to find many Krita tutorials on YouTube to help familiarise yourself with the software.
Do also remember you can select individual body parts in CTA4's Composer mode and then click "Launch External Image Editor" which will send only that part to Krita. You can then re-color and when you click "Save" the changes will automatically update in CTA4.