According to the provided information, I have created a sample project but I was not able to reproduce the observed behavior on my end.
I have attached my sample project for your reference. Could you please specify the exact steps how to reproduce the problem? Feel free to modify it in a way to reproduce the experienced issue and get back to me with it so I can investigate the precise case. Thank you in advance.
Currently, I can suppose to use the alpha component of the Color if you want to change transparent color of the RadPanel: