Hi Kori,
LAB colour seperates the image into 3 channels - a monochrome Luminosity channel and two colour opponent channels; (A) Magenta/Green and (B) Blue/Yellow. Normally when you mess with colour you're also messing with luminosity levels - but with LAB you can treat them seperately - which lets us do some things that are difficult if not impossible to do in RGB mode.
Basic technique is ...
- Image -> Mode -> LAB
- Add curves layer
- Switch to A channel and drag top end point left about 3 divisions (assuming a 10 division grid) (if you have only a 4 division grid then just alt-click the grid to change) then drag the bottom left end point right by EXACTLY the same amount (or you'll give it a cast).
- Repeat for B channel
- If it's a little "over-done" just reduce the layer opacity to suit
Do you have Photoshop? - thought you were a Coraldraw girl?