Here are three separate metaphors:

1. A linear continuum (like wavelength for light) from "no autism" to "really bad autism".

2. A collection of disjoint sets (like individual named colors like "cyan" and "puce") for cases like "really into trains autism", "freaks out at parties autism", "non-verbal autism", etc.

3. A continuous mixture of different properties (like rgb(.1, .2, .05)) for symptoms like "10% social dysfunction", "20% repetitive behavior", "5% sensory overstimulation".

When people describe autism as a spectrum disorder, they generally mean the third metaphor. It's a mixture of different symptoms and different autistic people have different amounts of those symptoms but all people diagnosed with autism have a significant amount of them and their symptoms will have some amount of overlap with other autistic people.

Number (3) has better explanatory powers than (1).

However, for the purpose of assessing social and family impact, it is rendered to (1). Both schools and state (US) programs use (1) to assess if a child qualifies for support. This is not always related to how to parent or educate the child.

Fortunately, the US school system with IEP (individualized educational plans) are developed along (3). (Source: two of my kids have ASD)

None of that necessarily helps in informal social contexts or in professional workplace settings. I think the American Disabilities Act covers reasonable accommodations for people with autism spectrum disorders, though I am not sure if it requires legal disabled status.

Lastly: I met a Native (Navajo) family with a child that seems to me, have some developmental disabilities — but I think they take a very different approach. For one, they don’t seem to have the usual social stigma associated with this, and are baffled why I would suggest getting state support for early childhood intervention. If anything, I would not be surprised if they thought I was, yet again, someone unthinkingly pushing a colonialist worldview.

> 1. A linear continuum (like wavelength for light) from "no autism" to "really bad autism"

This is the least helpful metaphor, when applied to anything with more than one dimension. "Really bad autism" can describe a multitude of unique symptoms.and is nearly information free, similar to describing someone as having "A really serious illness"

To take the rbg metaphor further, it should really be a "gamut" rather than a "spectrum".

I don't think the 3rd metaphor fits. rgb values still points to a single color, which maps back to a single value on a 0 -> 1 or red -> violet continuum. It's more apt to describe it like a multi channel audio mixer. Many different channels ("really into a specific topic", "freaks out at parties"), each with their own value (10%, 20%).

Metaphors often fail though, so it might just be best to say what we mean plainly.