Printheads are low on ink

Hello! What do event log lines "X-X printhead is low on ink" for all printheads mean? What can I do to get rid of them? I have print issues related to green tint appearing in light gray. Attached picture is from service manual.

Latex 315

#4 I've never seen those messages on Front panel. Does Latex 315 have color sensor? I have isuue with fading color during long printing. I've attached orginal image and printed one with issue. Issue started long before this printing and just before this printing i changed LM/LC printhead with a new one. As it shown on photo printer starts to print well and then magenta (or LM?) is fading during the printing and doesn't appear at later printing. I cleaned printheads, I printed printheads test - it is perfect.

All printheads are under 50% of usage. LC/LM is new. Ink cartridges are expired for 3 months but similar issue appeared about half year ago when ink were on warranty.