Keyboard key colour change

[Ofnuts]

Easy:

• Make a selection on the button, including its sides and base (ie, not just the top)  (I used the Path tool)
  
• Add a layer,
  
• Set to "Multiply" mode,
  
• Bucket fill the selection.
  

   

However, the devil is in the details. Notice how the green button is surrounded by a green reflection, while our button is surrounded by a white reflection. So we have to add that reflection too:

• Save the selection on the button (Select>Save to channel) (unless you have a path...)
  
• Make a selection which encompasses the reflection
  
• Select>Feather (I used 10px)
  
• Subtract the inital selection from it (Right click saved channel and 'Subtract from selection`)
  
• Add a layer under the layer you added for the button color, set to multiply
  
• Bucket fill the selection with the button color
  
• Reduce opacity (to around 40%)