The color that will be rendered by RadPdfViewer depends mainly on the color space. The color space is responsible for giving meaning to the color components. For example if the color space is Indexed and the color component is 1, then RadPdfViewer will transform this to an RGB color following some rules. In other words colors in PDF are not just RGB values.
There are some color spaces that are not fully supported by RadPdfViewer so this may cause some color differences. My guess is that this is your case. Here you can find a list of the most common features that RadPdfViewer does not support, including the color spaces.
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