I use small bars of soap on black or darks, It works great

USE school chalk

I have and it works really well. I save those motel bars of soap and sharpen the edges----and just keep sharpening as needed. I have never used any color but white (out of fear that the color might somehow stay) and all has always washed out nicely.

A silver marking pencil Berol silver works well, also one of the little chalk markers that have a wheel that deposits the chalk.

A piece of sharpened white chalk, works like a charm.

I have used sliver of bar soap very successfully. When it stops having a sharp edge, it goes back to the sink for a couple of days. Good as new.

I use a white or pink water soluble pencil, can be removed when you wash the finished work or just spritz with water and a light wipe with a small piece of batting. I find the chalk markers of all kinds don't stay on with all the handling my quilts get during construction.

i have used very thin leftover bars of soap to put an X on the wrong side....so i would consistently use the same side when making a project. lasts longer than chaulk. could put a small square of masking tape on the back.

I use a soap sliver a lot on dark fabric and it works well. A cheap white chalk pencil works for me too but the soap works better

I have used regular chalkboard chalk sharpened/ soap slivers/I accidently even went into my husbands stash of welding soap and that works well to. And if you are using a stencil just put some corn starch in a pouncer or a muslin bag and pat it lightly over the stencil.
Johanna