# How to know the exact colour after a level of transparency being applied?

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Salad Box on 27 Sep 2022
Commented: Salad Box on 3 Oct 2022
Hi,
I am filling an area with a specific colour, say [255 0 0 ], but with a level of transparency, say 0.5. So the actually colour that is going to fill the area is a lot weaker than a pure red 255 0 0.
If I use 'fill' to fill the area, then I
set(h, 'FaceAlpha', 0.5)
what colour do I get?
Is there anywhere I can find out this colour that actually being used to fill the area after 0.5 level of transparency being applied?

Jan on 27 Sep 2022
Edited: Jan on 27 Sep 2022
The color you see through a semitransparent element depends on the background. The rule is simple:
alpha = rand; % Anything between 0 and 1
Background = rand(1, 3); % RGB
Foreground = rand(1, 3);
Visible = Foreground * alpha + Background * (1 - alpha); % resulting RGB color
Salad Box on 3 Oct 2022
Brilliant. Thank you for your answers!! Very much appreciated!!

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