Here's a Logic Pro X feature that miss from working in Presonus Studio One. In the latter, you could select the scale/key you're in, and the piano roll would highlight only those notes for you, making it easy to work with the correct notes.

Logic Pro X has something similar with quantizing to a particular scale, but it wasn't as helpful...so I found a work-around....

This only seems to work with 10.1 and up, because you'll need the Collapse View button in the Piano Roll.

Pick a key/scale in your mind and draw a new, short MIDI region. Now, create a MIDI note for each note in the scale you selected. Rinse and repeat until the entire top-to-bottom piano roll is marked in that MIDI region. Select all the notes and mute them, and use this as your "template" for the selected key.

Now create a new MIDI region on the same track and select it, and also select the 'template' region. Press the Collapse View button (to the right of the Piano Roll 'View' drop down), so that your newly-created MIDI region will now be limited to only the notes you created in your scale/key template. With this method, you can draw & edit notes in the piano roll and always stay in key.

 

You can use the MIDI FX transposer to do this for you.

it wont highlight the keys on the piano roll but it will assign the key to your keyboard.