New monitor dosen't work when used as a second monitor

[Jackkay]

Replaced an old monitor with new HP Pavillion 27 monitor as a second monitor, When connected both don't work each on it's own works fine, monitor 1 is connected to graphics card with HDMI to HDMI monitor 2 in connected to graphics card HDMI to DP. When both are connected monitor 1 starts changing colors, white to red to green to blue, monitor 2 is lit up but nothing on display.

[TheOldMan]

Let's start with how the older monitor setup worked.  The old monitor that the new HP replaced - how was it connected? 

The monitor one - is that connected the same as it was before and is it the same as it was before?

This new HP Pavilion 27 monitor - please post the model number of that.