www.GIMP.org - Download GIMP and GTK
www.Wc3Sear.ch - Go to Tools and download Wc3Viewer
Use Wc3Viewer's TreeView window, searching BLP files to find the potion you want to edit. Then right click on it in treeview and extract and convert to tga. Now open it with GIMP and select the standard brush tool, change the painting type to Hue, and color to green/blue (health/mana). Now simply recolour the potions fluid to make it look like health/mana potion. Save as BTN<PotionName>.tga. Take the brush tool again, change Hue back to Normal. Now paint the border black again. Then select transparency, set it to 50. Now drag it over the ENTIRE icon ONCE. Save as DISBTN<PotionName>.tga. Open Wc3Viewer again, go to Convert Files in menu bar. Now convert both to .Blp (just press OK through all the pop ups about alpha channels etc).
Then import the icons either using a tutorial or the steps below:
1: Open import manager and import BOTH icons.
2: Change the path of the standard one to ReplaceableTextures\CommandButtons\BTN<PotionName>.blp
3: Change the path of the DISBTN to ReplaceableTextures\CommandButtonsDisabled\DISBTN<PotionName>.blp
4: Save the map, then give the standard icon to your potion, test the map and see if it works. If you did it right it should look just like you made it in GIMP.
5: Press F10, and look at your inventory, if the icon is green then your DISBTN path is wrong. If its green do step 3 again.