Hey there! Let's walk through how to nail this in GIMP step by step—no confusing multi-layer or multi-image hoops needed, just straightforward actions that'll get you exactly the result you want.

Your goal is clear: a pure green base, plus multiple red (R=255, G=0, B=0) and blue (R=0, G=0, B=255) selected areas. Here's how to pull it off smoothly:

1. Set the entire canvas to pure green first #

• Open your RGB grayscale image, then press Ctrl+A (or use the rectangular select tool to drag across the whole canvas) to select everything.
• Grab the Bucket Fill Tool (shortcut Shift+B, or look for the paint bucket icon in the toolbar).
• In the tool options bar at the top, set the fill mode to "Foreground color". Click the foreground color square, input R=0, G=255, B=0 for pure green, confirm, then click anywhere on the canvas. Your whole image is now the green base you need.

2. Create and fill red selected areas #

• Use the Path Tool (or Rectangular Select Tool) to draw your first red target area. If you used the Path Tool, head to the Paths panel and click the "Path to Selection" icon (the dotted square) to turn your path into a selectable region.
• Switch the foreground color to pure red (R=255, G=0, B=0), then click inside the selected area with the Bucket Fill Tool to fill it.
• Press Ctrl+D to deselect, then repeat this process for your second (and any additional) red areas.

3. Create and fill blue selected areas #

• Follow the same logic as the red areas: draw your first blue region with your chosen selection/path tool, convert to a selection if using paths.
• Set the foreground color to pure blue (R=0, G=0, B=255), fill the selected area, deselect, and repeat for all other blue regions.

Quick Fixes for Common Issues (Why Your Previous Multi-Layer/Image Attempts Failed) #

• Wrong layer selected: If you were using multiple layers, you probably tried filling on an empty or incorrect layer. Always make sure the main background layer (the one with your green base) is highlighted in the Layers panel before you fill.
• Path confusion: When using the Path Tool, either delete old paths after converting them to selections, or create a new path in the Paths panel for each new area—otherwise overlapping paths will mess up your selections.
• Fill mode misconfigured: Double-check the Bucket Fill Tool options: make sure "Fill area option" is set to "Selected areas", not "Similar colors" or another setting that might skip your target region.

That's all you need! You'll end up with a clean green base with all your required red and blue regions.

内容的提问来源于stack exchange，提问作者JohnW