Yellow roots

Hi all, hope you are well.

Please bear with me - as I'm new to gardening (having recently become a home owner) and know next to nothing about the subject - but I'm looking forward to learning 

Anyway, long story short, I was clearing out the front garden to make way for some new hedges / plants and came across an old tree stump. After digging around to try to remove it, I noticed that the skin under the bark, and the roots were a toxic yellow colour - the picture doesn't really do it justice:

I wasn't sure if this was normal, some sort of fungus or a method the previous owner had used to kill off the tree.

Any help is much appreciated!

Thanks in advance

Andy