U use a underlay of white when printing on black shirt?

if you want, you can use a white underbase, but depending on your ink, you can just do a pfp with the red.
Also I heard the new thing is to use a light grey color for an underbase in place of a white.

I have tried it both ways. Typically I have to do a print flash print flash print on deep reds on a black shirt. It's easier - in my opinion - to do this than to add the extra screen and go. Unless you have a large order or multi colors. It doesn't take much longer as they are "quick prints" on the second and final coat.
I am not a novice, but I am a perfectionist by nature and it looks better to me. When I started I would do a white print underbase, flash, then print 2 coats red.... both the same number of coats.
If anyone else answers they will probably tell you that you should ALWAYS print an underbase....I think that is the "legal" screenprinting law when printing red on blacks.

I only will print just red if i'm using an Opaque red.
Some standard Red inks are so transparent that you couldn't print them on darks period.. I have had this happen with Victory Factory Inks.

If the design has to pop I will underbase, but they will be made aware of the charges and the difference in color before I start.

I've done both ways with standard and max opaque inks. I perfer the white underbase although I have found a way to cheat. I take a white and mix with some red and curable reducer. This way when a underbase is choked you dont get that almost hairline looking stroke and only one red hit of red is needed. I tried the gray underbase and I only like this when doing a halftone underbase.

Depends on the colour. Definitly yellow, I frackin' hate yellow.