→Why scatter plots instead of colour merge images?: edit text to fill in detail and get it correct.
Most people might think that the image contains 4 distinct colours
prominent spirals are in shades of red and green, with yellow-green and yellow spirals in between, but the yellow and yellow-green actually have <b>exactly the same color!</b> You can verify this yourself by calling File>Open Samples>[[Spirals (Macro)]] in Fiji.
So... now, how do you feel about determining colocalization by looking for yellow blobs? Doesn't make much sense does it?
You need to measure something, not simply subjectively "look at" a red/green colour merge image.
An even better reason to always look at scatterplots / 2D histograms / cytofluorograms is that they actually show the thing you are looking for and talking about - the correlation (or not) between the intensities of the 2 colour channels of the pixels over space.