There follows some ideas as to how to use Photoshop to neutralise a colour bias to the background or colour cast to the main part of the image. Lighting pollution gradients, mis-matched exposures for the camera or filter set in use and a plain old dodgy sky can all cause unnatural colours in the image. Getting rid of them can be tricky if the gradient is different in each channel. If the background of your picture is uniform getting rid of the bias is just a few moments work.

Once again Curves comes to the rescue with the ability to make small changes in any part of the picture's brightness range. If the background is a bit green dropping the green channel curve at the lower end is often all that is needed. If the centre of your galaxy is the wrong colour work on the relevant curves at the middle to top end but be sure to anchor the bottom end of the curve to avoid introducing a new colour bias.

The first picture shows M63 with a green colour bias to the bottom left hand corner and an unnatural looking cyan colour in the centre of the galaxy. The first job is to sort out the background which, as the colour samplers show, is roughly neutral at the top right hand corner but green at the bottom left. Colour samplers can be re-positioned once in the picture and are removed by Ctrl and drag off.

As the green gradient is a straightforward one going diagonally from the corner to the centre it can be fixed easily with a quick mask selection and Curves. Drag the gradient from the corner at an angle that matches the gradient in the picture - practice makes perfect.

This and the next pictures are all rollovers, mouse over the picture to see the before and after effect. Quick mask gradients should generally be dragged twice as far as you think owing to the way the gradient works. Half of it will be less than 50% selected so the effect will gradually diminish. In Quick Mask mode you will see the masking values in the Info palette, when switched back to normal edit mode the numbers will show the underlying RGB values.

Below shows the selection active and the cursor is at the same point as the No1 sampler. Ctrl click will put that brightness value into the curve for the green channel. Using the arrow keys the curve can be brought down until the values in the No1 sampler are the same as for the No2. It may be necessary to make minute adjustments to the other channels, the read out will tell you. It is important to anchor the bulk of the curve to a straight line as no adjustments should be made to brighter areas of the picture. This is a background only adjustment. Mouse over to see the adjustment/anchor points.

With the background looking neutral the brighter parts of the picture can be tackled. What colour you choose is up to you and it can be difficult to know but in this case the central part of the galaxy is cyan and does not look right. An amber coloured aspect is probably about right. Adjustments are made as before, I am using a colour sampler here because it helps the demonstration, normally I would just hold the mouse steady and use the keyboard to change channels and place adjustment points on the curve.

The three original colour values are R=166, G=207 and B=211. A loose average of those values is 191, sticking closely to that should ensure that you do not raise of lower the brightness which would be the case if you simply adjusted say, Red, by raising it to 200. The red and green adjustments have left the picture slightly blue so a drop in the B value (mouse over) from 211 to 191 should be about right. All these curves now need to be anchored at the lower reaches to avoid the background changing colour. In fact, if you ctrl-click in the background for each curve you will automatically place the anchor and can then ctrl-click at the centre to make the adjustment. The curve should stay close to the original 45 degree line where you don't want to make changes but don't make it bend or kink sharply

Finally, the finished item. Mouse over for the before and after. Having dealt with colour balance and bias problems we can now get on with the job of making it look like a galaxy.