In Theros, there are three planeswalkers, as befits a large fall set, and all five colors are represented between them. Now, this means that no color "has" to be represented among the planeswalkers of Born of the Gods and Journey into Nyx - it's essentially a wide-open field. This is exciting, but, on the flipside, also makes speculation difficult. Fortunately, we have concrete evidence about at least one of the planeswalkers to be printed later in the block, and a strong basis for conjecture about the other. Let's take a look:
Born of the Gods: Yes, sea creature fans, it may finally be time for Kiora Atua. Ral Zarek finally got his first card this year, and Kiora was introduced slightly earlier than Ral was. The plane is right; Kiora would feel very at-home in a world full of Merfolk and that is literally touted as being full of monsters, no less in its seas than on land. Many Magic fans complained that all of the planeswalkers in Theros were designed in previously-used color combinations, and that neither green-blue nor green-white, the two-color combinations that are missing planeswalkers, got them, but it seems that the next set in this block is high time for a green-blue planeswalker.
Journey into Nyx: There's little doubt as to what character this planeswalker will be - the Journey into Nyx images that were discovered by accident along with many other Heroes' Path pictures show Ajani and Elspeth fighting together. All that remains is the question of his color combination; he can't be monowhite, because the current version of Elspeth is. He seems to have shed his anger and red side from when he was Ajani Vengeant, and many have speculated that he could change to green-white in his next version. Still, that would make him the third partially green planeswalker in the block, so I say it's a toss-up between red-white and green-white.