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If you're tempted by fixed I got a Hultafors HVK. Orange handle. Very cheap. Good sheath with a belt clip.
Would definitely stipple the handle a bit like I did here, but just do it all over. I'll try and update the photo.
Otherwise what about some bright string on a No.8 or 9? Actually the more I think about it, the more I think this is the right move. Especially harvesting.
Overall, though I think a folder is better. Especially for the uses you listed.
Edit: they do bright colours.
Or if you want to keep to the size of a No.8 for a bit more money
Want something cheap for the allotment: cutting string, cutting open bags and harvesting etc.
Obvious choice is Opinel but I do tend to lose knives so garish colour and/or pocket clip would be handy..