Why does the color empty out except for the black one

I assume we're talking bubblejet. Colour cartridges are generally smaller than the black because the black usually gets more use.

However, if the situation continues after you've applied new cartridges, there's a problem.

Printheads are not available generally though at a price (that probably exceeds that of a new printer) a repair tech could probably source and fit one.

To be frank, due to running cost and unreliability I abandoned bubble jet as soon as monochrome laser printers dipped under the $100 point.

And colour lasers are now approaching that region.