I'm not 100% what you mean so if I'm not answering right could you elaborate a bit?

Is it a semi-transparent colour on photoshop you're after?

Well from what I know of it you can either use the transparency slider on the top or do the colour on a seperate layer and change the layer transparency. Latter has the advantage over the former that it's easier to get a pure block with no streaks. I can get screenshots if you're really struggling.

Here's the deal: in paint, if you paste in a picture with the background the secondary color, and you click the 'transparent' button on the select area on the side, the secondary color will turn transparent. I need to know how to do it in Photoshop. Or whatever, im not entirely sure.

Photoshop does not lend itself easily for such stuff like that

There might be better people out there for answering this but honestly the only thing I can think of is either doing the art on a different layer from the background so it can be moved easily, or by erasing the background using the background eraser tool. Using a biggish brush - as long as the central point doesn't touch the thing you want to keep then it should get rid of the background for you. But it's quite a crude way of doing it.

Honestly it's easier to have the art on a different layer in the first place. They're automatically transparent and the background layer will remain untouched no matter what you do to this other layer.

Okay, i put the sprites on a different layer entirely...
Thing is, there's still the 'white background' behind it.
It doesn't work...

Oh so you copied the entire sprite sheet onto the layer. Ok.

welp you'll have to use the background eraser then. Make sure you click on the white background before you start erasing and keep it pressed. it has a little crosshairs in the middle so keep that out of the sprites and you should with any luck be ok.

That's the best I can offer you I'm afraid.

Mmk, and actually i'm not posting spritesheets, but individual sprites. But i think i'm O.K. for now...