Adjustment layers - curves

Hello, 

Adobe Photoshop does not allow for direct manipulation of spot colors in the Curves Adjustment Layer. In Photoshop, spot colors are typically used for specific applications such as package design, where a specific color needs to be maintained throughout the print process, and they are not part of the standard CMYK color model.

Here's a workaround:

1. Duplicate the spot color channel (Channels > Duplicate Channel). You can then treat this as an extra grayscale image and apply your adjustments to this copy.

2. When you're finished with your adjustments, you can convert this back into a spot color (Channels > Spot Color).

However, do note that the final output may not exactly match your initial spot color due to the conversion process.

If you want more control over spot colors in Photoshop, I suggest providing feedback to Adobe. They often consider user feedback when developing new features and improving existing ones.

In the meantime, Adobe Illustrator or Adobe InDesign might be better suited to handling spot colors, as they're more geared towards print and packaging design. If you are preparing files for print production, it might be worth investigating these other programs in the Adobe Creative Suite.

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Spot colors are pre-mixed to match the swatch samples from a company such as Pantone. They are not meant to be modified using levels or curves.  Here are the basics: https://www.shutterstock.com/blog/spot-colors-photoshop 

This is more feature request? Please post in Ideas section Photoshop ecosystem - Adobe Support Community