Painting for mudguard is needed!

Go talk to an autobody shop. If you really want to have it painted to last they should be able to prep, paint, and do all of that for maybe $150-$200.

They have all the base paints in stock as part of their day to day business so it is less costly for them to mix up the right amount vs. you having to buy the paint, sprayer, air compressor, have a clean environment to do it, clear coat, etc.

As a side note if in the US, try to find a shop that sprays PPG as they usually have a lifetime warranty with the job.

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Based on your update and not knowing what labor and quality of work is in India, or the value of the rest of the bike, you could always buy a can of spray paint, sand paper, spray clear coat to see what you can do that way.

Honestly, it probably will not look better than factory in terms of finish quality. The only way to get close to finish quality is take it to a auto body shop and even auto body shop may sometimes not have a perfect finish (e.g. orange peel, dirt in paint, fish eye, etc.).

You could also try Rhino paint liner paint that can be dyed different colors. I do not know if that is available in India. That is pretty forgiving for a nonprofessional to apply and it will end up looking the same as if a professional did it. A lot of off road SUVs use it on part or all of the vehicle because it is durable and cheap to repair.

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Update - I doubt you will find a solution for $7 dollars that will not look horrible or have any meaningful durability.

I'd suggest starting with enamel paints from a model shop. They come in small tins, and you will need one tin of each colour your design requires.

Enamel paints used to mean oil-based, but now there are "acrylic enamels" that can wash up in water, but still have oil based qualities.

Additionally you will need multiple small brushes, on a size to suit your design. Fine pointed brushes for fine lines and details, and some wider brushes (10-12mm) to fill in larger parts.

Preparation is key - literally. You need to prepare the surface so it will have a key, or tooth, to hold the paint. That means sanding, and you might require an etching primer too.

Lastly once your design is done, you will need multiple layers of clearcoat on top to protect the paint from impacts, dirt, weather, and fading from UV light.

Another significant issue is vibration and cracking - a bike is ridden and the mudguards/fenders will flex. This could crack the paint and make it fall off over time. I suggest you try a small test before investing a lot of time in something that may look awful after a few rides.

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Sadly one pottle of paint is around $4 USD so your budget won't go far. I'd suggest you buy some 220 grit sandpaper $3, one colour of paint $4, one spray can of clearcoat $12, and a paintbrush $2. There will be leftovers of course, but you're in for at least $20, or 1500 rupee. You might be able to source things cheaper, or perhaps you already have some items.

Either way, if you just slap paint on an un-prepared surface, it won't last very long. In that case, might be better to not do it at all.

Regarding 'water colour' paint job:

I have seen some water color paints that stick fairly well to metal. Probably not exactly what the OP is looking for, but with a bit of artistic ability and a few coats of a 2K clear coat, I bet you could make a pretty cool and unique paint job.

Get advertising and other stickers, cut them up and stick them on.