The "Retarder Medium" this slows down the drying time of the paint which gives you longer to blend colours together. I think this may be of use in larger scales and models (tbc). I do remember some painting blogs extolling its virtues.
A few drops of the "Matt Medium" makes the colour more transparent, increase paint (i.e. runnier) and produces a 'matter' finish. To me this has WWII 20mm (plastic and metal figure) potential to get a dirty camouflage look!
Lastly the "Metal Medium" again mixed with metal paint gives a paler metallic shade and can be also used for metallic highlights. I was thinking 25/28mm Wars of the Roses (Perry's), Renaissance (Perry's, Warlord Games) and other plastic 28mm ranges such a Normans, Vikings and Saxons - even up to Crusades (FireForge, Conquesr and Gripping Beast).
Watch this space for some results ;)