A series of rileys

Question

Riddle 1:

I’m a colour, but,
Come hell or my prefix,
I don’t care mon ami,

My suffix, though is literally nothing.
My infix is like Higgs, but royalty,

Take my fourth

First Letter:

E

Riddle 2:

My middle and start together,
Are a participle of a verb of wearing,
But my middle and end, is a bit of medicine,

My infix, is wasted time,
But my whole, means like a tree.

Take my fifth

First letter:

C

Riddle 3:

My prefix is I,
My suffix is a month,
My infix is a health tonic,

But I am boring and dry,
And naive, at that too,

Take my fifth

Riddle 4:

I’m slow, unmoving, I prefer to stay still,
And my prefix is your rank in the economy,
My suffix is the end of an explosive projectile.

My infix are the men at arms, But my whole describes plants

Take my seventh

First Letter:

S

Now what have you got?

Answer 1

Partial answer

Puzzle 3

Jejune

My prefix is I,

Je is I, in French.

My suffix is a month,

June

My infix is a health tonic,

Jun

But I am boring and dry,
And naive, at that too,

Jejune means both boring and dry and naive and simplistic

Take my fifth

N

Answer 2

Partial answer after the first letters were given:

Riddle 2

cladose (fifth letter: O)

My middle and start together,
Are a participle of a verb of wearing,
    — clad is an old past participle of clothe, still present in compounds such as ironclad.
But my middle and end, is a bit of medicine,
    — a dose is a quantity of medicine to be administred
My infix, is wasted time,
    — ado is a fuss about trifles
But my whole, means like a tree.
    — cladose means having branches

Riddle 4

sessile (seventh letter: E)

I'm slow, unmoving, I prefer to stay still,
    — sessile describes immobile organisms
And my prefix is your rank in the economy,
    — SES is an abbreviation for the socio-economic status
My suffix is the end of an explosive projectile.
    — –ssile is the end of missile
My infix are the men at arms,
    — the SS were the Nazi Party's men at arms
But my whole describes plants
    — sessile can also describe plants without a stem or stalk

Answer 3

Puzzle 1

Eerie black

I’m a colour, but,

Eerie black is a color

Come hell or my prefix,

The phrase is "come hell or high water". According to Wiktionary, "ee" is a noun meaning "water" in the language Tukudede.

I don’t care mon ami,

My suffix, though is literally nothing.

If you have nothing of something, you have a "lack" of it.

My infix is like Higgs, but royalty,

Peter Higgs, physicist, lives in Edinburgh, Scotland, UK. Something "royal" in Edinburgh is the Royal Infirmary of Edinburgh, or the "RIE"

Take my fourth

I

Full puzzle

So, taking @M Oehm's answer and @hexomino's answer into account, we have:

I O N E

I guess, after solving all those riddles, we were only left with

ONE I (one eye)

Answer 4

Partial answer: Riddle 2

ARBOREAL?

My middle and start together,
Are a participle of a verb of wearing,

This is the part I can't figure out.

But my middle and end, is a bit of medicine,

OREAL isn't exactly medicine, but such things are often grouped together ("health and beauty").

My infix, is wasted time,

BORE.

But my whole, means like a tree.

Indeed it does.

Take my fifth

R.

Musings on Riddle 4

I keep thinking this must be SEDENTARY, but I can't really make it fit.

I’m slow, unmoving, I prefer to stay still,

That certainly sounds like "sedentary".

And my prefix is your rank in the economy,
My suffix is the end of an explosive projectile.

Dunno about these lines.

My infix are the men at arms, But my whole describes plants

A man at arms could be a SENTRY, which almost but not quite fits in "sedentary". And plants are indeed sedentary.

Take my seventh

This would be A, which kind of fits with the preceding R and N.