You have text selected in a cell, and choose the Text tab on the right Numbers panel. You click the color globe, and choose RGB colors. You pick a color and your selected text obeys. Without closing the color chooser, you move to the next text cell, select the text, and click on a the same or new color. It will not change the color of the text in the cell. PITA alert!
You have to close the color chooser, and reopen it — each time that you want to change text color in a different cell. If you want to replicate the first cell color in the third cell, select the text in the first cell, and from the Format menu, choose copy style. Select the text in the third cell, and select Format menu : Paste Format.
This behavior is common with the color chooser in Pages v5.6.1, Numbers v3.6.1, and most likely Keynote v6.6.1.