RAM variables are automatically allocated by the assembler.
For instance, if you want to store your 0 or -1 in a variable named "color" all you need to do is start using "color" in @ commands. As long as is never appears in a label, it will be allocated in RAM. Similarly, you can use a pointer to the screen word you are clearing.
Examples (comments are C equivalents):
@color // color = -1
@color // *pScreen = color