Most color conversions were simply matrix multiplication. But you're right, the RGB -> HSV conversion assumes values in the [0, 1] interval. I will add a note. And I agree your 0-255 and 0.-1. defaults are sensible.

I still think it is dangerous to automatically scale images. To take the rgb2hsv function as an example, a way to scale would be to check the dtype and then if it is uint8, divide by 255. Now what if the image is 16bit? Or it was uint8 but the user happened to convert it to float already without changing the data range? Say the range is [0.5 - 127.], do you map it to [0. - 1.] or [0.5 - 127.] / 255?

When you need to assume a range, I think it should be in the docs, maybe even use a warning or a scale(=False) parameter if it's important. Provide some utility functions for users to scale to different data ranges. But automatic scaling is tricky and will usually not do the right thing for all users.

Cheers,
Ralf