Sadly, it doesnt do very well, as it eats up stack space
Yes, that is the drawback of flood filling, for larger areas it can soon exhaust stack space, the experience you have. Although it goes by 4 pixels at a time, is not very fast. One advantage, it can fill any shape.
As oposed to scan line fill, much faster especially when using BLOCK, and it can work with fixed size buffer. All good, except it may struggle with concave shapes.
On the genuine QL, I set aside 256 longs as static buffer for it.
Improved version of flood fill, may control stack usage, and revisit unflooded parts.
EDIT, Hi Per,
Knoware/toolkits/RPIX% does colour picking?