What could this be? Very prominent veins in penis, sometimes dark purple. Also spiderweb looking purple veins on top frontal part of penis?

Here are some ... All what you described may be the varying appearance of penile veins to reflect the degree of venous engorgement at different positioning and in the different state of sexual arousal. So, leave them alone as long as not hurting and not changing in the varying pattern of appearance. To assure the just described possibility, bring the info on the onset, degree, duration ... of these veins ... to Doc.