Start with priming with black primer. I used Scale 75 black primer with airbrush, but use whatever you have as it's not important for this piece.
For the upper half of the potion I used XV88 from Games Workshop mixed with a little bit of copper from Model Air to give it a slightly reflective surface and some kind of metallic noise to it. For this step I used my airbrush for speed, but it's not needed.
After the upper section was well dry I masked the upper section so I could airbursh the lower section. For this step I mixed Chocolate Brown from Vallejo, also mixed with a little bit of Copper from Model Air. If you are not using an aribrush you don't need to mask it.
When it’s dry, carefully remove the masking tape (if you used it) and paint the brain element using any highly reflective metallic paint. I used Speed Metal from Scale 75.
For the handles, the neck ring and the rivets I went for Dwarven Gold from Scale 75. At this point you can also blackline the separation between the upper and lower section with any dark paint with high fluidity. I used Inktense Black from Scale 75 for this step
When everything is dry add highlights to the golden parts and edge highlight all the areas on the handles and the rivets. For this I just mixed Speed Metal from Scale 75 with Dwarven Gold from Scale 75.
To add depth and saturation wash the golden areas with a reddish brown wash. I used Seraphim Sepia from GW
Start applying coats of Volupous Pink from GW to the brain. Use as many as you need to get good coverage.
After everything is done you just need to apply gloss varnish to the brain. I used Klarlack from Marabú. Almost any gloss varnish will do, but depending on the brand you will have to go heavier with the coats.