Adjust RGB levels to a flattened image.

Sorry if this is a really stupid question but how do you adjust the RGB Levels to a flattened tiff image? Is this something that would need to be done in lightroom or something? How can I adjust the numeric values of the seperate R, G and B channels to the whole image?

For bonus points... The image I have is going into a program called composer from gerber. When I print it in composer to a normal printer my print's blues are coming out as expected. When it goes to the plotter it is coming out very greenish. I can't seem to find anything wrong with the file, it looks right on screen and prints to an inkjet like normal but the plotter just hates this blue. I've tried darkening it and doing other things but it just doesn't work. Most other images work fine...

Thanks!!!

In either the Levels or the Curves palettes, you are able to adjust channels individually.

I was looking for a more basic plain simple RGB control over the image. For instance I want to reduce ALL the reds from the image with a very basic step.... the levels seems to be advanced controls with highlights, midtones and shadows.

Thanks!

Controls just like creating a custom swatch.

Wouldn't be better to use Hue & Saturation or even Selective Colour?

Have you looked at Image>Adjustments>Variations?