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Hultafors Chisel if you don't mind a fixed blade.
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Mora Companion might be an option.
Lots of very bright colours and a bit more useful than a standard pocket knife.
https://www.heinnie.com/mora-companion-magenta
You like your purple IIRC
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You can get Opinels in garish colours too.
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I put a Morakniv on my belt clip every time I step out the back door. Best £10 I ever spent.
https://www.greenmanbushcraft.co.uk/frosts-mora-bushcraft-training-knife-high-carbon-clipper.htm -
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..