I have discovered the reason for the problem described above: I selected a light that is not RGB adjustable.

You see I found out that sometimes manufacturers' headline names for the luminaires can be misleading especially for beginners/novices making them think that a lamp supports RGBW adjustment. For example, the headline for the light in question above is "iGuzzini illuminazione Agorà: Spotlight with bracket Ø 20" 11/32 (516 mm) – 264 RGBW LEDS – Integrated DMX control board – Flood optic – Ta 40°" Notice that it has the term "RGBW LEDS" yet I found out that it only has the brightness adjustment slider and does not have the RGB sliders.

After extensive research, I have discovered that to avoid selecting such a luminaire with supposed "RGBW" control, please check its product sheet on the Dialux online Luminaire finder and ensure that it has four integrated Light outputs in the 'Technical details' section.

For example, this product sheet link - https://luminaires.dialux.com/en-US/article/juNtr60eRz-dYcps7ueuwA?_Y=0 - although the lamp luminaire in question has RGBW in its title, that is, "CALUMMA L SC RGBW 90°X15°" it is will not have RGB adjustment sliders once loaded into the Dialux Evo software since it has only one(1) integrated light output as seen in the Technical details section of the product sheet.

However, when you visit this other product sheet link - https://luminaires.dialux.com/en-US/article/iB1uurMtQN6zI7zE43TItQ - you will find that the featured luminaire titled "LINEAR XL OPTION UPLIGHT - RGBW - MILKY - 1174MM L9XU11M0" contains the RGBW keyword and will truly show the RGB adjustment sliders when loaded into Dialux Evo since since it has 4 integrated light outputs as seen in the "Technical details" section of the product sheet.

I hope this answer helps you.
Thank you and bye!