I will never understand why they added a 'green' swirl to the vortex in seasons four and five. The vortex was always made of 'cold' colors (blue, silver, gray). If you wanted a green vortex, you should just create one...
It looks to me like the Season 4 vortex was a victim of shrinkflation.
The Season 1 vortex had flowing lines of energy that indicated the specific direction of the vortex and had a transparency at the edges that required blending into and warping the image of the surroundings. The silver glow effect required multiple layers of composition and animation for both the perimeter of the vortex and the internal body as well as rendering the vortex at different angles. The Season 1 vortex was effectively a rippling hole in the skin of reality, often resembling a bubble that was partially bursting to allow entry.
The Season 2 vortex, however, reduced the transparency ripple at the perimeter severely while the silver light was now filled in with blue. This appeared to be, in some ways, a cost or time saving measure. It was no longer necessary to blend the vortex into the shot with lens distortion effects because the transparency was reduced and the interior of the vortex was now filled with blue light rather than a transparent silver. The Season 2 vortex was less an aberration in reality and more a doorway to wonder.
The Season 3 vortex benefitted from a higher special effects allocation: the animation of the water-whirlpool effect of the vortex was far more intricate and detailed than the first two seasons. The silver colour of Season 1 was restored and and a thin level of transparent edge distortion. Also, the lines of energy charging into the vortex took on a more nebulous but varied and denser; it looked less like straight lines and more like flowing water.
So what happened with the Season 4 vortex? Like the Season 2 vortex, they simplified it. They simplified the perimeter: the transparent edge effect was only present on specific shots, so there was now no effort to blend the vortex into the surrounding shot.
They simplified the body of the animation: rather than lines of energy or a mass of flowing water, it was now a rotating spiral of cloudy energy composed of grainy dots, an effect that, if nothing else, was captured well by the grainy format of the 16mm film of the Sci-Fi Channel years.
And with so little animation and a vaguely defined body texture and no transparency blending, the vortex in silver-blue would have just looked like a blurry coin or a puddle on the video, so to fill in an otherwise blank void, they made it a hypersaturated blue and green designed to contrast against the shots because they no longer had the time, money or inclination to blend and coordinate the vortex to match the shots.
It's rather sad because the vortex is a vital part of SLIDERS' iconography: the timer, the basement, the wise Professor, the funny soul singer, the sweet shorthaired girl, the flannel clad boy genius, and the rippling, glowing tear in reality that is a gateway to adventure. But by Season 4, the gateway looked less like a tunnel and more like somebody spilled blueberry Gatorade on the lens.
I am a Gatorade Zero person myself.