If you are still using NX Client 3.5.0, you may set  support for high and true color mode in RDP sessions via the NX Client GUI in the General -> Use custom settings -> Settings panel.

Please note that, starting from version 3.0.0,  the way NX manages RDP sessions is changed. Further information for NX Server 3.5.0 is available at:

https://www.nomachine.com/AR06E00469