As this seems to be a reocuring question I thought it might be convinient to have a post to refer to. Keep in mind I'm not a chemist, but this is how I understand this all works . This is meant as a simple explenation so I'll try to keep it short and to the point. Transparants and opaques Transparants: These paints will alway's show the color thats beneath it, meaning it will blend with it. As such they can be used to mix colors on ones painting instead of mixing it in a cup. A transparant can never reach full opacity so if you want a yellow line on red it won't work as it will create an orange line. Opaques: These paints behave more like one expects of paint in that they will just cover/ color with the color used. These colors will reach full opacity. If one puts a yellow line on a red surface it will do just that. With enough reduction an opaque might apear to become transparant. This is just down to the factone is hardly applying any pigment anymore, with enough layers a highly reduced opaque will still reach 100% opacity. An opaque is made by adding white or black to paint. This results in that in general a transparant will be more vibrant as they are not diluted. Adding a bit of black or white to a transparant will make it opaque. This is also the reason there is transparant black and white as one can't chance the hue of a transparant with an opaque black or white (as that would result in the transparant turning opaque). Reducing and viscosity Below is technicaly not completely (completely not ) correct but explains it for as far this is relevant to airbrushing. Viscosity: Viscosity is important to understand for reduction. When we inmagine paint as consisting of a lot of little balls (paint and pigment), viscosity refers to the fact that all these little balls sticks together. Due to the fact they all stick together we get a nice and even aplication of the paint. Where all the little balls to break appart and form little blotches of paint we speak of paint losing viscosity. A: paint has viscosity, this results in the nice and even spread of color (B) as every bit of surface get an equal amount of pigment. C: paint has lost it's viscosity, this results in a "blotchy" spread of color (C) as some spots on the surface will get hit by pigment and others not. Reduction: There are two main reasons to reduce paint. -Enhance the flow of the paint -Make the build up of the color more slow (generaly to et a bit more control) When we are painting we are aplying pigment to a surface. This pigment dictates how fast the color will build up. Unreduced paint could be said to have 100% pigment. The moment we start reducing it's important to keep in mind we also reduce the % of pigment so even when one reduces just to make the paint flow better a slower build up of the color is an automatic result also. Why is there transparant base and reducers (water) As the two main reasons to use reduction at all would be both covered with water (or reducers) which would reduce the amount of pigment and make the paint less thick, whats the use of transparant base? Here is where viscosity comes into play. When adding a lot of water at a certain point the paint will lose its viscosity and break up. This results in one spraying water with the occasinal blotches of paint where there where enough "paint molecules" near eachother to create enough viscosity to make that blotch of paint. So one can't endlesly add water to paint. Transparant base (name of this might differ as for the brand used) is paint without pigment. As it is of the same consistency as the paint (only doesn't have any color) this will in no way affect the viscosity of the paint. When to use what? If one wants to make paint thinner water or reducers are used. Depending on the paint this can be done by a certain amount before it loses viscosity. When this point is reached one has just hit the limmit of the paint, nothing to be done about that, it won't get any thinner. If one wants to reduce the amount of pigment water can be used as above (will ofcource also make the paint thinner). Another option here is how ever transparant base. This reduces the amount of pigment without reducing the paints viscosity (as one is just adding paint without color). If one is reducing with water to reduce the amount of pigment and hit the point where paint losing it's viscosity transparant base is the only solution to reduce the amount of pigment even further A last remark on thinning paint: Thinning paint will not affect the size/thickness of the pigment. this means some paints just won't go through (or clogup) the smaller nozzle sizes (0.15-.02) nomatter how much reducer is added as they contain particles that are just too big. Especialy metalics or flakes come to mind here.