Right, on the GH games the notes which you cannot HO/PO have black rims. On RB the notes you cannot HO/PO are larger. It's harder to tell them apart when star power is activated, but they are different there, too.
On medium you should be able to strum just about every note.
I've got two recommendations for Chad for medium difficulty:
1) Try doing some "alt-strumming" (alternately pressing the strum button up and down) on passages which you have a sequence of the same note. It's not necessary for passing (or even 5-starring) medium, but it will help you later.
2) Get used to moving your hand from what I call "first position" (index finger on green and pinky on blue) to "second position". (index on red, pinky on orange). Again, not strictly necessary for medium, but useful. You can practice this when you have a green note followed by a red. Play the green note normally with your index finger, then shift your hand slightly and play the red note with your index finger, too.