अथ यदेतदादित्यस्य शुक्ल भाः सैवर्गथ यन्नील पर कृष्णं तत्साम तदेतदे-तस्यामृच्यध्यूढं साम तस्मादृच्यध्यूढंसाम गीयते ॥ ५ ॥
In plain words
Now, whatever white radiance belongs to the sun is the ṛk, and whatever dark or deep black light there is, that is the sāman. This sāman rests upon that very ṛk, which is why the sāman is sung as resting upon the ṛk.
Close to the Sanskrit
Now, whatever is the white light of the sun, that indeed is the ṛk. Now, whatever is dark, further, black, that is the sāman. This sāman is founded upon this ṛk. Therefore the sāman is sung as founded upon the ṛk.