Hi Ashok,
Thanks for contacting Apple Support Communities!
It sounds like when you connect your Dell monitor to your Mac mini and change the color profile to RGB, the Dell monitor displays in green, correct?
To start, check out this resource to help you to determine if the Dell monitor is support by your Mac: If your external display is dark or low resolution - Apple Support
"Find out what your Mac supports
Your display will be dark if it exceeds the maximum number of displays that your Mac can use at the same time. This limit varies by Mac model.
Your display will be dark or use a lower resolution or refresh rate if you configure it to exceed the maximum resolution or refresh rate that your Mac can use. These limits vary by Mac model, and by the resolution and refresh rate of other connected displays.
To find out how many displays you can connect, and at which resolutions and refresh rates, check the specifications for your Mac model:
If so, we'd suggest that you reach out to Dell to investigate this further with you. They'd be best suited to help you with your monitor. You can find their contact details here: DELL Technologies Support
Take care!