Why is the injection site yellow after taking methotrexate? it looks like someone colored it with a high lighter

It's yellow.: When reconstituted Methotrexate is yellow in color. You probably have some under the skin.

Bruise by injection: Bruise from injection may change color due to the breakdown of hemoglobin from red blood cells in extracellular space. Colors of a bruise are caused by sequential degradation of hemoglobin to biliverdin to bilirubin to hemosiderin, with hemoglobin itself producing red-blue color, biliverdin producing green color, bilirubin producing yellow color.

It's yellow!: Check out the picture! Methotrexate is very yellow. I assume you are not having it injected into a vein. If it's injected im (intramuscularly) or sc (subcutaneously) the yellow color will seep into the skin. If you are having it injected into a vein, your skin should not be yellow. This means it's not getting into the vein!