1. Make sure The Optifine mod is installed.
2. Navigate to your Minecraft Install folder, Most likely at C:\Users\[USERNAME]\AppData\Roaming\.minecraft.
3. Open the mods folder, and right click on the Optifine .jar file.
4. Click "Open with WinRar."
5. Double click on the assets folder to open it.
6. Open the folder called "Minecraft", then open "mcpatcher"
7. Now open "Colormap." If it does not exist, just right-click empty space, and click "create new folder", and name it "colormap"
8. Inside the colormap folder is where you will need to add redstone.png
9. To create the redstone.png file, open up mspaint.exe, and resize the canvas so that it's 16 pixels wide, and 1 pixel tall.
10. each pixel will represent the colour of each level of redstone strength. Left pixel is full-strength, while the right pixel is zero-strength. Between these 2 is usually a gradient.
11. Give these pixels whatever colour you want, and save the file somewhere as "redstone.png"
12. Now go back to WinRar, and click the "add" button in the top-left corner, then select the "redstone.png" file you created.
13. It should now be set up correctly. Just close WinRar and run the game, place some redstone, and take a gander at your new colours!
I made my redstone dust white, so it can be used for road lines in my city: