How to change color of disable state of default "Submit" button in captivate knowledge chek?

Please, I need some more information. What do you mean by 'default "Submit" button'? That default Submit button can be linked to a quiz slide or to a Text Entry Box, to give some examples.  If it is a custom button, it is pretty easy to give you a possible workflow, but if it is an embedded Submit button on a quiz or KC slide it becomes difficult. Moreover you cannot even disable such a button. You didn't mention neither which event you use to disable the button.

For a custom button two possible workflows:

1. Create a custom 'Disabled' state, and add the command 'Change State ....' to the Disable command which means you'll need an advanced or shared action. Moreover you didn't specify how and when you want to go back to the Normal state.

2. If you don't use the InBuilt state 'Visited', you can create that state and it will automatically appear when the button is clicked. But since I ignore the rest of your setup, I'm not sure if this could be possible.