I have a simple two color gradient on an image.
I changed the colors to pantone colors to print the design. But now that I changed the colors, the gradient lightens up in the middle.
I would love an explanation why this happens or a way to prevent this.
It's because you can't blend a Spot color (Pantone) to another Spot color.
When you blend from spot to spot the only option Illustrator has is to use the Document Color Mode (CMYK/RGB) for the blending. That results in generally a "grey" tone. It's a bit more prominent in CMYK mode, but it's there in RGB mode as well.
For Spot to Spot gradients you need 2 gradients.....
Color A 100% @ 100% opacity -> Color A 100% @ 0% opacity
Color B 100 % @ 0% Opacity -> Color B 100% @ 100% opacity
Then you stack them on top of one another, either via multiple fills in the Appearance Panel or by using multiple objects. And lastly, set the top object's fill to overprint in the Attributes Panel.
Prior to AI having the ability to use opacity on gradients, the gradients were merely created with 100% color to 0% color, then one fill was set to overprint. That still works as well. It won't look correct on screen (especially if you fail to turn on Overprint Preview), but it looks correct off press. Using opacity simply provides a slightly better on-screen preview.