I have made an interesting discovery while messing with my copy of mIRC.
I don't exactly have a step process aside from 'I opened my client and connected.'
I use my own custom theme color, not the monochrome, and I don't even use that color in my theme anywhere, for the bars to show up that way.
But, as I said, an interesting discovery. I am using a laptop, with a monitor hooked up as a second display.
Primary Laptop Display:
Secondary Monitor Display:
As I move the mIRC window from screen to screen, the weird color glitch appeared or went away.
I do have two video cards in my laptop, an integrated Intel HD 630, and then a high processor nVidia 1060ti. The HDMI port to my laptop is a connection straight into the nVidia, while the main laptop screen is primarily (but not in all instances) handled by the Intel HD 630.
As it is, the Intel HD 630 should be handling the visual processing of mIRC on both screens, as it is not a video game, but I am no expert on how laptops with multiple video cards behave.
Also, your echo result is in the screenshots.