Transparent Background Text with Secondary Color Feature

Hello,

I absolutely love how the font and background are interacting here. I'm curious what's determining how the colours are responding to the background.

For example, if you look at the image below, where "Afterhours" overlaps green in the bottom images, the grass is a different shade of purple than how the kelly green "Fresh" turns fuscia). 

There's also some inversion effect happening.

Thank you!

https://afterhoursstudio.com.au/ Sorry forgot to add the link!

From the screenshot it looks like it is using an invert filter.