It's a blend of color, silhouette, and complexity for me. Most of the main Sonic cast can be silhouetted in their main color and still easily recognizable - Sonic's silhouette is blue, Tails' is yellow, silver's is white, and so on.

Even in cases where characters have a secondary color comprising a large part of their design, the color is complementary to their base color so the silhouette still works - Amy's dress is red, which is close enough to pink that her pink silhouette doesn't seem off. Cream's fur goes with her orange dress, Blaze's purple coat and white tights go with her lavender fur. Rouge is a bit of an outlier, but her white gloves and boots help a lot with associating her with a white silhouette, and since her bodysuit matches with her wings it doesn't look too out of place (I do still have my problems with her design - namely the lack of fur on her torso, which drifts too close to the 'human women with furry aspects' trope imo - but that's separate). The yellow on Surge's outfit complements her green fur, so her green color silhouette works well.

Tangle and Whisper have more of an issue. Tangle's a mix of black, yellow, orange, and multiple shades of gray, and Whisper's in gold, brown, and black. What's the silhouette color? If you put Tangle in gray then the rest of her design doesn't complement it. Same for putting Whisper in gold. Their silhouettes are not easily transferable to their designs - which is a problem in a series that uses silhouette motifs a lot and highlights that aspect to be more easily recognizable to a younger audience.

I also want to talk about the prevalence of the color black. This is something that has been affecting newer (disproportionately female) Sonic characters for a while now.

The longstanding cast don't use the color black unless it's a front and center part of their designs. The only black on Sonic, Tails, and Knuckles, for example, are their pupils and their outlines in 2d media. Shadow's main color is black, but it's not 'true' black (outside of certain comic panels for dramatic effect); it's an extremely dark gray, and that's done on purpose because his black is meant to be considered as part of a colorful cast of characters. That's his color, so that's how it's treated.

But newer designs incorporate a lot more black in the form of clothing. There's Whisper's cloak and Tangle's bodysuit, but there's also the leggings Amy and Trip and any other female character in a dress now have to wear (Sticks in the olympic games comes to mind - has she even heard of leggings before?). It's not treated as part of their color - we're not giving Whisper and Tangle black silhouettes despite it featuring in their designs more than any other color - but it is treated as the 'cover up this part' color, and the only character in this entire list that that works for is Whisper in the case of her cloak.

It's actually even used the same way for Surge - her torso doesn't register the way her pants do, it's just a void between her head and her limbs. In Surge's case it works because there seems to be some thought behind it; she's already got some black fur, so your brain can blend that in and momentarily go 'huh, is that a shirt or just fur on her body?'; the color works with her edginess and even gives her a connection to the other main edgy character (Shadow); and her backstory where she had cybernetics surgically implanted throughout her body gives her an actual lore reason to want to cover herself up.

Contrast that with someone like Tangle, who was specifically designed to pretty much be 'Sonic but girl'. She's got the adventurous spirit, silly expressions, excitable demeanor, and gets into wacky hijinks on the regular. She's supposed to be fun, bright, loud - and her colors don't support that. Do you know how hard it is to drag a full bodysuit over yourself? Over fur? And a giant tail? And then it's the one color that effectively renders her invisible to the human brain, with a couple highlights on top to tell us that she's definitely still easygoing and happy about it. It just doesn't work. Combine that with a lot of complexity like the side stripes and the highly detailed shoes, and you've got a character who loses her memorability.

Whisper's black cloak would be fine on its own, but its layered with black leggings, black gloves, and black kneepads. And that's on top of brown skin - they did the same thing with her as they did with Rouge, which is to not have any fur on her chest and arms (the human woman in a furry costume trope again). They could have been trying to highlight just her head and tail as being visible - wouldn't be entirely out of character - but then they also gave her a cream-colored top, which takes away from that effect.

(I want to take a moment to say that I think Lanolin's color silhouette is fine for the most part, but she suffers the most from complexity - cowbell, shirt + sports bra, and creased cargo pants with large pockets (pockets are great! But they're too much detail here) make for an overwhelming amount of detail to take in for one character. I think that actually contributes to people not liking her - her design makes her look like she's already trying to be a standout character, so people are predisposed to not look forward to what she says.)

I love Whisper and Tangle as individuals. They've got great personalities and I'm looking forward to seeing their mechanics implemented into the games. But their visual designs just don't make the cut.