Thanks for your query
Large lump may be because of hematoma
that has undergone bruising and sequential colour changes. I think your knee swelling spreaded gradually and ultimately reached your ankle because less tissue support over your sheen bone. Yellow discolouration is due to sequential colour changes of your bruise
. Nothing else. Though i must ask you to have a x-ray over your trauma site to exclude fracture
. Though chance of fracture is very less here but we must exclude it. Apply nothing over that yellowish portion because it will disappear spontaneously.