I am trying to change this plot

to match this style

Changes required are as follows:

1. change colour of individual boxes for oxide to red, transition to green and fresh to blue
2. add legend for oxide, transition and fresh
3. Simplify x-axis to include only the elements

Here is an excerpt of my data:

Oxidation   Elements    GAI
1.OXIDE Ag_ppm  1
2.TRANS Ag_ppm  1
2.TRANS Ag_ppm  0
2.TRANS Ag_ppm  2
2.TRANS Ag_ppm  2
2.TRANS Ag_ppm  1
3.FRESH Ag_ppm  2
3.FRESH Ag_ppm  0
3.FRESH Ag_ppm  0
3.FRESH Ag_ppm  1
3.FRESH Ag_ppm  0
3.FRESH Ag_ppm  0
1.OXIDE Ag_ppm  0
1.OXIDE Ag_ppm  1
1.OXIDE Ag_ppm  0
1.OXIDE Ag_ppm  0
1.OXIDE Ag_ppm  0
1.OXIDE Ag_ppm  0
1.OXIDE Cu_ppm  1
2.TRANS Cu_ppm  1
2.TRANS Cu_ppm  1
2.TRANS Cu_ppm  1
2.TRANS Cu_ppm  2
2.TRANS Cu_ppm  1
3.FRESH Cu_ppm  2
3.FRESH Cu_ppm  1
3.FRESH Cu_ppm  2
3.FRESH Cu_ppm  2
3.FRESH Cu_ppm  2
3.FRESH Cu_ppm  1
3.FRESH Cu_ppm  2
1.OXIDE Cu_ppm  3
1.OXIDE Cu_ppm  3
1.OXIDE Cu_ppm  3
1.OXIDE Cu_ppm  4
1.OXIDE Cu_ppm  2
1.OXIDE Mg_pct  1
1.OXIDE Mg_pct  1
1.OXIDE Mg_pct  3
1.OXIDE Mg_pct  2
1.OXIDE Mg_pct  1
1.OXIDE Mg_pct  1
1.OXIDE Mg_pct  2
1.OXIDE Mg_pct  2
2.TRANS Mg_pct  2
2.TRANS Mg_pct  2
2.TRANS Mg_pct  2
2.TRANS Mg_pct  2
2.TRANS Mg_pct  2
3.FRESH Mg_pct  2
3.FRESH Mg_pct  2
3.FRESH Mg_pct  2
3.FRESH Mg_pct  2
3.FRESH Mg_pct  2

Here is my current script:

# Plot boxplots
fig, ax = plt.subplots(figsize = (20,8))

# Box plot properties
mean = dict(marker = 'x', mec = 'Black', ms = 9)
median=dict(color ="Black", lw=1.2)
whisker=dict(color ="Black", lw=1.2)
cap=dict(color ="Black", lw=1.2)
flier=dict(markerfacecolor= 'green', ms=10)

key_label = ['Ag','Cu','Mg']
key_ox = ['Oxide','Transition','Fresh','Oxide','Transition','Fresh','Oxide','Transition','Fresh']
col_ox = ['r','g','b','r','g','b','r','g','b']

gai_waste.boxplot(ax=ax, by = ['Elements', 'Oxidation'], column =["GAI"],
                  color = '#ADD8E6', patch_artist=True, whis=[10, 90], #widths=0.2,
                  whiskerprops = whisker, showmeans=True, meanprops = mean, medianprops=median, capprops = cap, showfliers=False,
                  grid = True, rot = 90, fontsize = 'medium')

plt.ylabel("GAI", fontsize = 'x-large', fontname='Calibri', weight='bold')
plt.xlabel(None)
plt.yticks(fontsize = 'large', fontname='Calibri')
plt.xticks(np.arange(1, 10, 1),labels=key_ox,ha='center',fontsize = 'large', fontname='Calibri', rotation = 0)
ax.yaxis.set_major_formatter(FormatStrFormatter('%.0f'))
plt.grid(True, which='both', color='lightgrey',ls = '--')
plt.suptitle('')
plt.title('')

x=2
for i in range(3):
    plt.text(x = x , y = -5.5, s = str(key_label[i]),horizontalalignment='center', fontsize = 'x-large', weight='bold')
    x+=3

Thanks in advance for your help!