I saw the promo of Missing Milo and I didn't notice much differences in the color style, except for the scene starting at 3.87 seconds. Actually I didn't like how they colored it, but I thought it was because of the time vortex they were in. The rest of the promo doesn't show big colour changes.
About the second season, I hope they don't mess too much things up, because I liked the style in Phineas and Ferb, and I keep loving it in Milo Murphy's Law. I'm kinda used to it, so a big change, especially in a cartoon which has a whole story behind itself like Milo Murphy's Law, would make it feel... uncomfortable. A slight change instead could make it better, like when they made Ice Age 2: they used the same graphic style, just used better programs to render it. In fact I liked more Ice Age 2's graphic than its prequel.
As said, I think Milo Murphy's Law's graphic is already awesome, and trying to make it different is risky. But let's wait to judge, because if the "changes" are like the ones I saw in the promo of Missing Milo, then I probably won't even see them.