Text Color

One way to distinguish a Title from a Subtitle could be to make the Title text completely black with #000, vs. a Subtitle of the same size and weight, with the color of #687684.

Text Weight

Similar to color, varying weights can give different interactive definitions to the text used in the design. In Instagram’s example, they are using an incredibly subtle font-weight change to indicate actionable profile names. Whereas profile @mentions and #hashtags are treated with color instead of weight because they are created by user input instead of the Instagram design system.