RGB means red, green and blue light. In the additive model, adding these lights can produce white. Paints and inks absorb parts of incoming light instead. Value means how light or dark something appears. Label colors so meaning is not carried by color alone.
Worked example
Red and green light together can appear yellow. This does not tell us that red paint mixed with green paint will make yellow paint: pigment results depend on the materials. Always name which mixing system you mean.
Two Different Materials
In an ideal RGB display model, full red plus full green plus full blue produces white, and all three at zero produces black. A printer commonly uses cyan, magenta, yellow and black inks. For an offline experiment, make three pencil patches labeled light, middle and dark; notice value without needing colored pencils.