Do you mean oxalis? (There are both yellow and red-leaves varieties of this plant.) Or do you mean knotweed (Persicaria capitata, Polygonum capitata)? It is a deep-rooted ground cover with red and green leaves and pink flowers. Some varieties of Persicaria have more colorful leaves.

Many thousands of plants are grown in Southern California gardens, and an invasive plant having “red and yellow leaves with roots about 18 inches long” is not enough information for plant identification.

But one thing is clear, don’t use Roundup! Whatever the problem, just pull it out or kill it with soil solarization (described on pp. 295-296 of my book). Follow up with an organic non-poisonous pre-emergent herbicide, such as corn gluten meal, to prevent its return from seeds. Roundup is a poisonous chemical product that has been shown to be more dangerous and more lasting than previously supposed. It lasts in the environment and can invade groundwater.