Prob a recolored white to transparent gradient decal on top of a nein part.
As @oddcraft18 already mentioned you can use a decal or what I thought of is that you can use a beam placed in front of the window part. From there, make it fade from bottom to top.
Depending on your needs, I use a quick method to get that effect for purely decorative non-functional windows on exteriors. I use a smooth plastic ‘shiny brick’ with reflection set to 1 in the place of glass. Shiny bricks reflect the skybox like a mirror so the color of the ‘glass’ will be whatever the skybox is.
I was able to emulate the picture above using a shiny brick with a little bit of transparency backed up with a thin pane of white neon block for a glow and a ‘sunset’ skybox.
- Disclaimer - the shiny brick method has worked best for me with a static time of day.
For day/night cycle games I use regular semi-transparent glass, a gradient decal on the glass and make the neon backer whatever color I want the window to glow.
Quick links if you need them:
Shiny brick: https://create.roblox.com/marketplace/asset/10100297/Shiny-Brick
Gradient decal from this set: https://create.roblox.com/marketplace/asset/13168370735/Modular-Building-Kit-Modern-City