Why will my brick not change colors?

Although it might look like it, the text you see at the BrickColor properties in Studio are not strings, but BrickColor values. The proper way to create a BrickColor value is through...

BrickColor.new()

This is the function which gives the Parts color. There are 3 (4) types of values that this function accepts, but we'll stick to the string. All you need to do for the string part is to insert the string within the parentheses, like this:

BrickColor.new('Really red')

Therefore, to apply the "Really red" color, you use:

B1.BrickColor = BrickColor.new('Really red')

The exact same method is used for "Lime green". I recommend you take a look at the following Roblox Wiki pages (which will be more useful than what you might think)

More than you ever wanted to know about BrickColor

A full list of available BrickColors (There are more than you can choose in the palette)

local B1 = workspace.Deathrun.Button1
local walk = workspace.Deathrun.walk1
local walk2 = workspace.Deathrun.walk2
function onClick()
    B1.BrickColor=BrickColor.new'Lime green'
    walk2.CanCollide = false
    walk.CanCollide = false
    print 'off'
    wait(10)
    print 'on'
    walk2.CanCollide = true
    walk.CanCollide = true
    wait(7)
    B1.BrickColor=BrickColor.new'Really red'
end
--We have a part so we get the new value by equaling it to ".new"
B1.ClickDetector.MouseClick:connect(onClick)

If this helped please accept my answer and upvote if possible thanks it's much appreciated.

local B1 = workspace.Deathrun.Button1
local walk = workspace.Deathrun.walk1

local walk2 = workspace.Deathrun.walk2

function onClick()

B1.BrickColor = BrickColor.new('Really red')

walk2.CanCollide = false
    walk.CanCollide = false

print("Off")
    wait(10)
print ("On")

 walk2.CanCollide = true
    walk.CanCollide = true

wait(7)
    B1.BrickColor = BrickColor.new('Lime green')

end

B1.ClickDetector.MouseClick:connect(onClick)