The colour red appears quite often in angiosperms but in various shades. Think of all the pink, purple, red and blue flowers, the bright fruits and the deep shades of autumn leaves.
This is due to anthocyanin. Anthocyanin is a vacuolar pigment that occurs in all tissues of angiosperms including the leaves, stems, roots, flowers and fruits. Think of all the red berries and fruits there are that range in colour from dark blue to bright red. Strawberries, peaches, blackberries, rose hips.
The most obvious advantage of this to the plant is dispersal. Both flowers and fruits are vital for reproduction and dispersal, therefore being a bright colour helps the creatures they depend upon find them easily.
As for the stem, roots and leaves, anthocyanin seems to have an added extra advantage. It helps aid against UV damage. This is apparent in the fact that young shoots tend to show a purple tinge, as this is when they are most vulnerable to damage, when energy is being invested in growth rather than protection. This is also true for maple leaves in autumn. Before deciduous trees lose their leaves, they extract the nutrients from them and so at this time they are vulnerable to UV. Anthocyanin acts as protection enabling the tree to complete its extraction process and giving this tree its famous deep red shade in the autumn.