I have designed a custom logo to go on a black headstock of a guitar. I have text colored with a goldish color. RGBA is 210, 190, 0, 255. The opacity is 100%. The logo looks great when printed out with an inkjet printer onto a white decal sheet. When I slide the decal off the sheet, and onto the black headstock it disappears.

Any ideas?
thanks

Do you mean that it disappears entirely?
Probably too obvious, but could you have printed on the wrong side of the decal?
Maybe the text just needs to be larger or bolder? If the text has a black stroke, maybe try removing the stroke? Or maybe try some different shades. Sometimes a color looks very different against a white background, than it looks against black (or vice versa)(although probably not disappear...). I've just tested your color, and I can see where it might not show up well against black. I found r - 250, g - 220, b - 0, a - 255, possibly a little better, although I realize it's entirely your personal preference. If you haven't already, try setting the color with a black background in Inkscape (change it in Document Properties), or at least just put up a quick black rectangle behind the logo, so you can see how it looks.

Oh, maybe the decal needs to dry, to bring out the true color??? Well anyway, that's about all I can think of. I wish I could be more help

I can tell as the decal slides off the paper it is too transparent. Somehow I am not getting enough color "down". I tried printing in fine mode instead of normal, and still didn't get enough ink on the decal to show up on the black surface. I keep thinking my settings or layout in Inkscape are not right.

EDIT: Well I learned on a guitar board that I need to use white back transfer paper instead of the clear. And then use a black background in Inkscape to blackout every thing outside of my graphics.

I hope those tips will help.
I was also going to comment that perhaps your printer just isn't capable of the quality you were expecting?