I would not say the tryptamine CEV's are dimmer, but it may of course depend on the compound. CEV's on 4-AcO-DMT are not dim for me. On 25-30 mg 4-AcO-DMT it's everything but dim. I'm not a big friend of phens but I've used some, inlcluding 2C-I, but 4-HO-MiPT has produced the brightest and most beatiful visuals of all tryptamines, beating all phens I've tried too. Also, you are comparing two very different psychedelics. 2C-I is very energetic while 4-AcO-DMT is very mellow, for me very sedative and almost dreamlike.
One reason you may conceive the CEV's on phens as brighter are that the nature of the visuals are quite different from tryptamines. Phens give me quite "technical" CEV's, more machine-like, more "matrix-like". I know we're talking about CEV's here but on DOB my OEV's consisted of neon coloured webs, boxes and rectangles, something that has never happened with a tryptamine. So you also have to take into account the nature of the visuals, phens have different visuals that have a different impact on you than tryptamines. I think it's a matter of perception, not that phens "really" produce lighter visuals. And phens are generally more stimulating, making you more alert, while tryptamines can feel almost dreamy.
What other tryptamines have you used? It would give a better understanding of why you think phens have brigters OEV's than tryptamines.