Hi,
Set COLOR effect to (number) block states that color values to be used are -100 to 100 range.
When I write a loop to draw and number the colors, I get the display below.
Where are all the greyscale and black and white colors ?
I know it is possible to set them via the set COLOR block with the color picker option.
But I have no idea how to derive the requisite NUMBER to create them programmatically.
Also, as one can see, -100 to 100 range actually repeats the colors !
Basically, the number is just adjusting the Hue of the colour - to get the complete range of colours, there needs to be a parameters to adjust the Saturation and Level as well
Snap just follows the simple way Scratch did it.
Sorry ignore most of that 
Snap has options to change the saturation and brightness as well as colour (hue)
The next release will have color dimensions like the Scratch 3.0 ones.
Your question shows how confusing it is to use the name "color" for the hue. We've had some arguments about what to call that dimension, because "hue," which is the correct technical term, isn't in the vocabulary of a typical teenager, our target audience. What it really means is "the nearest rainbow color to your color." Suggestions for a better name for that would be welcome!
The way you get grays is to set the saturation to zero.
We are still arguing about the third dimension, for which there are two candidates. (Look up "HSL and HSV" in Wikipedia.)
But what you should really do is load the "Provide 100 selected colors" library, which makes this all really easy.
Thanks for the explanations. Now I understand better.
