Fresco assigns all colors similar values when viewed in greyscale .... does this make sense?

[hakru]

Colors have tone values, which can be judged by viewing color art in greyscale .... . I noticed that Fresco does not reproduce this expected behaviour, when I put down colors with different hues (but same saturation/brightness) and view them in greyscale (using a color adjustment mask or setting the iPad display to greyscale) - the show essentially the same tone value. This confuses me and spoils the use of tonal studies in learning coloring & art with Fresco. Any ideas/chance to  correct this?

[hakru]

Edit:

I was wrong about the iPad display to greyscale setting - this produces the expected  greytone value variation among the color spots in Fresco. However, my attempts to use a mask to obtain the greytone values of the color spots directly in fresco with the iPad display in color mode fail:

To obtain greyscale in Fresco, I add a filter mask ("Hue/Saturation") and set "saturation" to -100, leaving Hue & Brightness untouched (0). Applying this mask to an imported color foto yields the expected greyscale image., applying it to the layer with the color spots shows them all in a very similar, unified greytone. That's very different from what I see for the spots with the greyscale setting for the display.