How Spec Maps Work in The Sims 4

A spec map, or specular map, is the layer of an item that tells the game how shiny or matte it is. The Sims 4's spec map is broken up into four channels: red, green, blue, and alpha (opacity). Each channel will produce different effects in combination with the others or by itself at different strengths. These matrices were produced by adjusting only one variable at a time and applying it to the t-shirt then seeing how it looks in game and taking a screenshot. The strength of color shown in the background is approximately the color the spec map looked for each shirt. 

The alpha layer (see top matrix) adds a reflection or sheen at between 12.5% - 87.5% opacity of the colored channels. It’s more liquid looking than metallic and seems most effective at around 50% strength. 

Green effects the brilliance of the shine making items appear to be made of polished metal. At low opacity, it still implies a metallic quality to an object such as a brushed metal tap. 

The way that blue effects shine makes an object seem curved and plastic-y. Probably best for lacquered effects like automotive paint or nail polish. Would be lovely at 50% strength and 50% opacity on Art Deco furniture for that high-glamor look. EA uses the blue channel on plants for a waxy look.

Red makes things look smooth and can create effects from vinyl and latex at high strength to silk charmeuse and crepe on low opacity. 

For a sparkly effect, stipple white and green on a black background. You can make a very nice lip gloss look using blue and red in combination. In combination, just about any effect is possible. High opacity, high percentage spec maps are very extreme, so use caution when you want to add shine.