With only a JPEG to go off of, it is hard to be sure on the forum (we do not have access to all of the information you do, only what you post) - but it looks like you have a two channel image. So you are picking up the green cells if you choose the green channel, and the red cells if you pick the red channel. Unless you have a third yellow channel, you are not picking up yellow cells, just green cells that also happen to have red in them.
Without a marker that is in 100% of the cells, you do not have anything for cell detection to work off of. It is a similar problem to trying to measure how much green is in a green cell using the green channel. You cannot do that appropriately since you can’t use a channel as it’s own area threshold (plenty of other posts on similar topics).
You may want to approach this a different way, measuring area based off of a pixel classifier, or using a simple thresholder and some scripting. At the moment though, it sounds like you are trying to use the cell detection inappropriately. If I misunderstood and you do have a channel that is present in 100% of your cells, then please do correct me.
Another option might be creating a channel (in FIJI for example, or via scripting) by adding the other two channels together… but that will likely run into intensity issues if your signal to noise ratio is low or your channels are too different in terms of intensity.