Here is a link to the documentation explaining what the options and syntax are for changing line properties (including color) in plot (semilogx has the same options and syntax for this).
Below is how you might make those two plots red:
depth = xlsread('942 Raw data.xlsx', 'Sheet1', 'I:I');
bulkdensity = xlsread('942 Raw data.xlsx', 'Sheet1', 'L:L');
graindensity = xlsread('942 Raw data.xlsx', 'Sheet1', 'N:N');
porosity = xlsread('942 Raw data.xlsx', 'Sheet1', 'O:O');
voidratio = xlsread('942 Raw data.xlsx', 'Sheet1', 'P:P');
data = [depth bulkdensity graindensity porosity voidratio];
%%
data = sortrows(data,1);
depth = data(:,1);
bulkdensity = data(:,2);
graindensity = data(:,3);
porosity = data(:,4);
voidratio = data(:,5);
%%
depth = [0;depth];
N = diff(depth);
a = (bulkdensity - 1.024)*9.81.*N/1000;
Ov = cumsum(a);
%%
figure(1)
plot(porosity,depth(2:end),'r.');
axis ij
xlabel ('Porosity(%)');
ylabel ('Depth(m)');
hold on
%%
figure(2)
semilogx(Ov,voidratio,'r.');
xlabel('Sv');
ylabel('voidratio');
hold on
