have you looked under Options?
Better to try under...
Express pull down>
Tools>
System variable editor
(Or the Express panel on the ribbon has the same Tools tile contining the System Variable Editior - if you use the ribbon)
There you will find...
Gripdyncolor
Gripsize
Those will control what you are asking about and more. The size is universal for all grips though, not jiust dynamic block grips.
Hope that helps and I hope you see the value of the system variable editor that the Express team came up with.
If you find that this is an answer to your issue please mark as solved.
Ron
For LT users:
Options>User Preferences>Block Editor Settings..
There you can find colors and sizes for grips and parameters etc..