Citadel Technical Lahmian Medium is a colourless medium used to thin Citadel paints for glazing, layering and smoother colour transitions. It helps reduce the strength of a paint without simply flooding the mix with water, giving more control when building transparent layers.
Add a small amount to your chosen paint on a palette, then adjust the mix until the colour is transparent enough for the job. It is useful for soft shadows, subtle colour shifts, smoother highlights and fixing transitions that look too harsh after the first pass.
Different colours become transparent at different rates, so test the mix on a palette or spare model part before using it on an important area. Lahmian Medium is there to give you more control over consistency, not to replace judgement at the brush.