I lift very heavy, about 4-5 times per week.

Past few months I have noticed across my biceps there are many red stretch marks... they seem to get more colorful each time I lift and plentiful.

One friend tells me because I lifted too hard and too fast, another friend tells me it is a good thing because it means muscle is expanding and that I should be happy about it.

I'm hoping the ladde ris correct thanks for opinion

I think there stretch marks, i have them myself on my biceps but there white now. I got them when i was agressively doing curls in my late teens.

So yeah i think its due to growth.

do you think it is a bad thing though?

The marks? Yeah i wish i never had them. The growth? NO.

Not much you can do about it now, i doubt, just continue to train hard.

You might try cocoa butter. I started using it when I began to notice marks around my pec/delt region a few months ago. They have only ever showed up during workouts/when pumped.

I began using cocoa butter immediately, and they haven't progressed at all. Can't guarantee that it'll reverse pre-existing marks, but it's worth a try.

do you think it is a bad thing though?

Ehhh I've got TONS of them in the armpit area. You just live with them. They eventually fade to being the same color of your skin and hardly look different. They are just signs of hard work. Don't sweat it.

WHen I reached the levels of morbid obesity with my stomach stretching past more than it could, I was covered in bloody red strech marks all over. I should mention I have sensitive and fairly light colored skins, so it looked horribile. I did the following to prevent any more strecth marks and diminish the ones I have:
1.) Applied Aveeno Triple Oat Moisturizing lotion at night and Vaseline Extra Moisturizer with Vitamin D lotion in the morning 7 days a week. This is something you should get the habit of doing regulary and for the rest of your life. Not only does it keep the body moisturized and healthy for years to come but my girlfriend just loves the smooth feeling of it :)
2.) Exfoliate all the dead skins on your body (strech mark area) regularly.
3.) Get sunlight exposure. A tan can completely make the diminishing strech marks look completely obsolete.
4.) Lose body fat slowly
5.) Drink lots of water to ensure your body's well hydrated.

Within a month the current strech marks went lightish white....Its hardly noticable now