Click the Blend mode button in the Colors dialog to open the Blend Mode dialog.
The blend mode affects only the pdf export. Blend modes are used to determine how two colors are blended into each other.
OCAD supports the blend modes Darken und Multiply. The blend mode is only visible in the exported pdf file, not in OCAD!
More about blend modes in Wikipedia.
The Darken blend mode is set for the colors Purple and Green over Brown.
The blend mode is not visible in OCAD.
After exporting pdf file the blend mode is visible when opening this pdf file in Adobe Reader. The colors Purple and Green over Brown are transparent.