Is it ok to 'grey out' disabled buttons? | by H Locke | Medium
Designing Button States – Cloud Four
Disabled Button designs, themes, templates and downloadable graphic elements on Dribbble
Testing a Button Component
Adam Silver on Twitter: "Don't disable buttons. If a user makes a mistake it won't be clear what they've done wrong. Always let users click the button, and show an error message
Design Tip #10 – Don't Use Disabled Buttons - UX Design World