There's got to be some confusion here, because it is a very purple spice and while it doesn't dye things as aggressively as say turmeric does the tint it adds is very much purple. I've used it in curry before when I didn't have lemon or vinegar to give it the little bit of acid it needed to really bring out the flavor, and it tasted amazing but looked completely grey because the purple of the sumac and the orange of the turmeric clashed. I personally don't care about the appearance of my food, but other people were definitely hesitant to try that particular curry because of the color.

I've also added it to tea that I was brewing and it added a clear purple tint. When mixing it into baked goods it also stains the dough purple a bit. Looking it up it's historically been used as a red or purple dye, so the exact hue of it may vary by species (like chinese sumac looks like it has much paler fruit than the mediterranean variety from the pictures on wikipedia, so it may have a different color when dried and ground too) or by how fresh vs dried the fruits are.

Other considerations aside, natural dyes are notoriously fickle things so it's going to come down to what else the sumac gets mixed with.

Probably the most relatable example of this is if you ever used red cabbage juice as a pH indicator - those changes in pH make a huge difference to the colour that the juice shows.

I wouldn't be at all surprised if sumac imparted a deep red or purplish colour in enough concentration (when in the right situation - e.g. sumac lemonade) but I've never seen za'atar turn dough or labneh or vegetables an unusual colour so idk. I'm no expert and I've never looked into it deeply but I am a sumac aficionado and I'm handy in the kitchen so make of that whatever you will.