Dialog Colors & Shades Depend on Active Sprite ColorMode

Hello,

I observed this in Aseprite version 1.3-beta5.

I found this out because I'm working on a custom new sprite dialog which lets the user set their own color profile. But I believe it would impact any dialog with a color widget or color shades widget that could be opened (intentionally or not) independent from an `activeSprite`.

To Reproduce:
- Create a new sprite.
- Set the new sprite's color mode to gray.
- Open a dialog with a color picker or color shades widget.

The color picker will display only grayscale colors. The rules for when or if the color picker is updated if I, say, close the gray scale image and open a new RGB image isn't clear.

Because script dialogs can be opened independently of sprites, and because dialogs have the capacity to create new sprites, the behaviour I would prefer is that the dialog's color picker and shades be independent of the current sprite's color mode as well.

Thanks for considering this.
Jeremy