Were Bryan Garner (A Dictionary of Modern American Usage) to have edited Northrup’s work, he would have been just as likely to write, “It is difficult to tell whether the red, white, or black is predominant.” He submits that predominate is a needless variant for predominant.
While the meanings of the two words are nearly identical, there is a rationale for the differentiation.
Predominate – to hold advantage in numbers or quantity; to exert controlling power or influence
Predominant – having superior strength, influence or authority; being most frequent or common. (M-W)
Predominate is best used as a verb. Predominant, however, is always used as an adjective. Both words are used correctly in this sentence:
In the current Congress, Democrats predominate (verb) in the Senate, while Republicans are the predominant (adjective) party in the House of Representatives.
Both words are formed from the root dominate whose meaning is to master or rule and whose verb and adjective usages are clearer. I trust few of us would stumble over these word choices:
Ranchers’ interests dominate the Western Governors Conference agenda.
The dominant concern of the Western Governors Conference is ranching interests.
Both predominate and predominant can be turned into adverbs by adding ly, but predominantly predominates in good usage.