First, no it is not too late to pull a tissue sample, unless your beans are already podding. Even if they are flowering, I would go ahead and pull a tissue sample.

Are your symptoms across the whole field,  or only in spots/ sections of the field?  If only in spots or sections, I would consider (2) tissue samples - one where the plants are showing yellow, and one where they are not showing yellow.   In addition, I would consider pulling soil samples in the same areas - one soil sample from good area, and one from yellow area.  then, you can compare the results of the soil samples, and corresponding tissue samples, and see if that tells you anything.