This is the giveaway! The stem is usually yellow but heavily "flushed" or streaked with bright red or reddish-brown.
While Zeller’s is fairly easy to identify, never eat a wild mushroom unless you are of its identity. boletus zelleri
If you bruise the yellow pores or cut the flesh, you might see a faint, slow shift toward a blue color—a common trait in many boletes. Where and When to Find Them This is the giveaway