Why are Beast and ''Ultra Instinct'' the same color?

[Zebra]

It's clearly supposed to be an evolution of Ultimate. That's why Gohan was in his Ultimate form when he unlocked it.

On the one hand....
Toriyama had every opportunity to mention Ultimate when explaining what he had in mind with Beast, and he didn't.

Toriyama was only speaking about the form's design. Out of universe, it's inspired by SS2 Kid Gohan. In universe, it's not a Super Saiyan form at all.

For instance, Goku starting out a fight in Blue before turning on Ultra Instinct wouldn't mean that the latter is rooted in the former, it would mean that he started the fight at one level of strength and decided partway through the fight that a higher level was needed. And if I remember correctly, Gohan goes from Super Saiyan to Ultimate in the film, and I don't think anyone's going to try to argue that Ultimate is rooted in Super Saiyan.

The first time Gohan unlocked Ultimate, he was in his base form, and we know Ultimate is rooted in base.

Likewise, the first time Gohan unlocked Beast, he was in his Ultimate form, so ...

Maybe Ultimate is rooted in Super Saiyan, at least in part? In the manga, the Elder Kaioshin tells Gohan to power up like he's turning into a Super Saiyan in order to turn Ultimate on.

I see that as more Gohan releasing close to his full power. Before turning Ultimate, Gohan had to turn Super Saiyan to release his full power. Ultimate became his full power when Elder Kaioushin powered him up, so he turns into that instead.

[Zephyr]

Toriyama was only speaking about the form's design.

No, he wasn't. This is not a comment about the design:

Gohan has achieved his own unique evolution, based off the sort of awakenings he had as a boy. I've even named this transformation "Son Gohan Beast", in the sense that the wild beast within him has awakened.

This is a comment about the design:

For the design, I tried drawing him with a scary face and pale skin, but somehow this didn't feel like Gohan. So instead I just gave him the usual big upturned hair...

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The first time Gohan unlocked Ultimate, he was in his base form, and we know Ultimate is rooted in base.

Okay I don't think I'm being clear enough. When I'm calling a form "rooted" in something, I mean that it's a prerequisite, it's the source of the power, etc. That he was in his normal state when he became Ultimate doesn't mean it's "rooted" in his normal state, in the way I've been using the word.

Unless you are meaning to say that you think it is "rooted" in his normal state, in the way I've been using the word. Which wouldn't be correct, since we know it's "rooted" in 魔.

Either way, interpreting Ultimate as a Super Saiyan form was meant to steelman your argument. If Ultimate is rooted in Super Saiyan, then Gohan going from Super Saiyan to Ultimate in SH does not prove troublesome for your argument that "going from Ultimate to Beast means that Beast is rooted in Ultimate". But if we dismiss the notion that Ultimate is rooted in Super Saiyan, then Gohan going from Super Saiyan to Ultimate in SH does prove troublesome, because it means that Gohan going from Ultimate to Beast while fighting Broly doesn't work as an indication of a connection between them.

[Zebra]

Toriyama was only speaking about the form's design.

No, he wasn't. This is not a comment about the design:

Gohan has achieved his own unique evolution, based off the sort of awakenings he had as a boy. I've even named this transformation "Son Gohan Beast", in the sense that the wild beast within him has awakened.

Not only about the design, touche, but those "awakenings" refer to his rage boosts, not specifically Super Saiyan.

But if we dismiss the notion that Ultimate is rooted in Super Saiyan, then Gohan going from Super Saiyan to Ultimate in SH does prove troublesome, because it means that Gohan going from Ultimate to Beast while fighting Broly doesn't work as an indication of a connection between them.

Sure, but he was clearly Ultimate right before transforming for Gokuu.

[Zephyr]

those "awakenings" refer to his rage boosts, not specifically Super Saiyan.

I know, that was my original point. Beast would only also be rooted in Super Saiyan if both:
1. Ultimate was rooted in Super Saiyan
and
2. Beast was rooted in Ultimate
But...

he was clearly Ultimate right before transforming for Gokuu.

...again, I don't think he is. I think he is clearly in his normal state there, going off of the second panel of this page where his hair is messy like in his normal state. I could understand if you think that's Ultimate in the sixth panel where he has the enclosed eyes again (ala these panels of Gotenks mid-Super Saiyan 3 transformation arguably depicting "Super Saiyan 2 Gotenks"), but to me that reads like he has the enclosed eyes because Beast is being activated.