One of the most useful tools in Gimp is the Colour Select tool, which allows you to select all of one colour and those similar to it in one click. The main application of this tool is to replace colours in an image, but what about images where you want to replace a range of colours? In this Autumnal photograph the trees at the top of the image are a range of oranges and reds, yet those at the bottom are still green. If I wanted to replace those no amount of fiddling with the threshold setting on the Colour Select tool would allow me to only select the green leaves; I'd always select too much of the image and colour the branches by accident, or I'd not select enough and green leaves would show through. So here, I'm going to show you how to replace the colours in such a scene without having to spend forever playing with the threshold to no avail.
Step 1. Open your image and press the New Layer icon in the Layers pane. Name the layer something simple like ‘1'; we only need to know which layers have already been colour in this project, so I'm just proceeding through numerical values for my layer names.
Step 2. Now left-click the Background layer so that it is the active layer and select the Colour Selector tool by pressing Shift and ‘O'. In the Tools pane there will be a set of options opened up now; most importantly here the Feather Edges and Threshold settings. You need to experiment here; we want the threshold high enough for each selection to include enough colours to make a visible difference to the image, but we do not want them to include areas such as the tree trunks. You may have to alter the settings for this tool for each colour you replace, but you will always want a slight feathering on your selections to make sure you cover all the areas between the colours that may not get selected since it is very hard to select every colour in the range that you wish to replace.
Step 3. Select a colour, preferably starting with the darker shades of your colour (then progressing up into the highlights in successive layers) and then once you have checked that your Threshold setting is ok, you can left-click the layer you created before. Press ‘P' to select the Paintbrush tool, set the size to maximum and select a corresponding tone in the colour you want to replace the original with. Fill the selection that you have active in this layer and you will fill all your selections in this layer.
Step 4. Create a new layer and repeat this process of selecting colours in the background layer and filling new layers with colours to replace all the colours you want in the image.
Step 5. Now, go to each layer in turn and change the layer mode. Depending on the effect you want you will need to try a variety of modes, but for my image Soft Light allows the depth from the original image to show through and still colours it, so it seems the best choice. Once that's done you have your finished image.