Better solution:
For idx = 1 To 10
Eval("Bb_Swing" &idx &"Ext").Brightness = pfBright
Eval("Bb_Swing" &idx &"Ext").GlowBrightness = pfGBright
Next
For idx = 1 To 20
If idx > 10 Then Idx = 20
Eval("Bb_Bonus" &idx &"Ext").Brightness = pfBright
Eval("Bb_Bonus" &idx &"Ext").GlowBrightness = pfGBright
Next
But I would like to know why Execute doesn't work and Eval works...