How do I fix the pink tint in the menue bar, windows, icons of a 7500/100 Power Mac system 8.6?

I'd try disconnecting and reconnecting the display cable to the computer's video-in connector several times. That thing is really old and there may be some corrosion after so many years.

Is this an after-market monitor with a video adapter between the display cable and the computer? If so, do the plug/unplug thing with that several times as well.

If the monitor is attached to a PCI display card instead of the logic board display connector, remove and reseat the video card.

I don't know which monitor is attached but many old Apple monitors had adjustment knobs to allow adjusting the color balance. Some were on the front panel and some were recessed in the back and required a plastic screwdriver to safely adjust. I used to melt the plastic barrel of a cheap, disposable ballpoint pen into the shape of a screwdriver tip for those. Worth a try.

You could also go to Displays/Monitors control panel and use the Color setting to recalibrate the monitor.

Worst case scenario is that either the green or blue electron gun is kaput, causing the red to take prominence. Adjustment can't fix that.

This old article reminded me of the "hold OPTION" trick to access some control panel color settings:

Macintosh 21" Color Display: Adjustable Color Temp

I just checked one of my 8600s running OS 8.6 to see if the Appearance control panel has an option for creating a custom theme with a pink tint to windows and it doesn't. The fact that the pink tint is pervasive inclines me to believe that the display you're using may have (as Allan suggested) developed a problem with one of the color guns. If it's a malfunctioning Sony Trinitron (single-gun), repairing it is likely not feasible. A non-Apple display may have internal adjustments for color balance, but this assumes that the gun(s) are functional and that you'd have access to service manuals. You may need to find another, older CRT display to use with your 7500.

How badly do you want to get this machine up and running? If you could find an original Radeon for Mac PCI card on eBay, you could hook the 7500 up to a more recent monitor. The Radeon card has to be the Mac edition, and it has to be the PCI card, not AGP.

Pretty good sleuthing. Not sure what video card you mean: the machines of that series that I used had 4 VRAM slots rather than a stock graphics card. Of course you could also add a PCI graphics card, which was what I ended up doing with all of them.