x:y means, that this attribute has at least 2 or more channel functions. Channel functions are displayed in the column to the left of each encoder.
3:0 means, that at moment the value 0 from the third channel function is active. Unfortunately, we cannot see the Red attribute encoder in your screenshot.
As you can see in your second screenshot, the same value is displayed in the encoder. It is in my opinion only logical, that the same value will be displayed in the fixture sheet and on the encoder.
In the column to the left of the Red encoder, the third channel function is activated, which explains, why the encoder and the fixture sheet display the „3:“ in front of the value.
Strange, that only the red attribute has several channel functions, but not the other color attributes.
At the end the fixture type should be checked, whether the channel functions on the red attribute are not needed, or if the other attributes miss them.