Button highlight subpictures on a DVD are indexed, 2-bit color, which means only one color and opacity value is possible per index. You can have three separate values (e.g. =1, =2, =3) in Encore, and mix and match them, but effects like blurs, glows, drop shadows and gradients are not possible using standard DVD subpicture button highlights.
You can do these sorts of effects if you have a separate menu for each button state, but it's a lot of linking work if you have more than a few buttons. Also, these type of menu-to-menu buttons are generally slower than standard button highlights using the subpicture layer, and some DVD players switch from menu to menu more slowly than others.